# On Polynomial Multiplication Complexity in Chebyshev Basis

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1 On Polynomial Multiplication Complexity in Chebyshev Basis Pascal Giorgi INRIA Rocquencourt, Algorithms Project s Seminar November 29, 2010.

2 Outline 1 Polynomial multiplication in monomial basis 2 Polynomial multiplication in Chebyshev basis 3 A more direct reduction from Chebyshev to monomial basis 4 Implementations and experimentations 5 Conclusion

3 Outline 1 Polynomial multiplication in monomial basis 2 Polynomial multiplication in Chebyshev basis 3 A more direct reduction from Chebyshev to monomial basis 4 Implementations and experimentations 5 Conclusion

4 Polynomial multiplication Arithmetic of polynomials has been widely studied, and its complexity is well established: Let f, g K[x] two polynomials of degree d < n = 2 k and K a field. One can compute the product f.g K[x] in O(n 2 ) op. in K [schoolbook method] O(n 1.58 ) op. in K [Karatsuba s method] O(n log r+1 (2r+1) ) op. in K, r > 0 [Toom-Cook method] O(n log n) op. in K [DFT-based method] assuming K has a n th primitive root of unity.

5 Polynomial multiplication Arithmetic of polynomials has been widely studied, and its complexity is well established: Let f, g K[x] two polynomials of degree d < n = 2 k and K a field. One can compute the product f.g K[x] in O(n 2 ) op. in K [schoolbook method] O(n 1.58 ) op. in K [Karatsuba s method] O(n log r+1 (2r+1) ) op. in K, r > 0 [Toom-Cook method] O(n log n) op. in K [DFT-based method] assuming K has a n th primitive root of unity. Remark this result assumes that f, g are given in the monomial basis (1, x, x 2,...)

6 Polynomial multiplication in R[x] All these methods works on polynomials over R[x]. most of them use evaluation/interpolation technique O(n log n) method needs to perform the computation in the algebraic closure of R, which is C, to have primitive roots of unity.

7 Polynomial multiplication in R[x] All these methods works on polynomials over R[x]. most of them use evaluation/interpolation technique O(n log n) method needs to perform the computation in the algebraic closure of R, which is C, to have primitive roots of unity. Let f, g R[x] with degree d < n = 2 k, the sketch to compute h = fg is: set ω = e 2iπ 2n C and calculate [f (1), f (ω),..., f (ω 2n 1 )] C n using DFT on f and ω [g(1), g(ω),..., g(ω 2n 1 )] C n using DFT on g and ω fg = f (0)g(0) + f (ω)g(ω)x f (ω 2n 1 )g(ω 2n 1 )x 2n 1 C[x] [h 0, h 1,..., h 2n 1 ] R n using DFT on fg and ω 1 plus scaling

8 Discrete Fourier Transform DFT Let f (x) = f 0 + f 1 x f n 1 x n 1 R[x]. The n-point Discrete Fourier Transform (DFT n ) can be defined as follow : n 1 DFT n (f ) = (F k ) k=0..n 1 such that F k = f j e 2iπ n kj C which is equivalent to say j=0 DFT n (f ) = (f (0), f (ω),..., f (ω n 1 ) with ω = e 2iπ n.

9 Fast Fourier Transform FFT The Fast Fourier Transform (FFT) is a fast method based on a divide and conquer approach [Gauss 19th, Cooley-Tuckey 1965] to compute the DFT n. It uses the following property: Let f = q 0 (x n 2 1) + r 0 and f = q 1 (x n 2 + 1) + r 1 s.t. deg r 0, deg r 1 < n 2 then k N s.t. k < n 2 f (ω 2k ) = r 0 (ω 2k ), f (ω 2k+1 ) = r 1 (ω 2k+1 ) = r 1 (ω 2k ). This means DFT n (f ) can be computed using DFT n 2 (r 0) and DFT n 2 ( r 1), yielding a recursive complexity of H(n) = 2H(n/2) + O(n) = O(n log n).

10 Polynomial multiplication in R[x] Asymptotic complexity is not reflecting real behaviour of algorithms in practice, constant and lower terms in complexity does really matter!!! Exact number of operations in R to multiply two polynomials of R[x] with degree d < n = 2 k in monomial basis Algorithm nbr of multiplication nbr of addition Schoolbook n 2 (n 1) 2 Karatsuba n log 3 7n log 3 7n + 2 DFT-based ( ) 3n log 2n n + 6 9n log 2n 12n + 12 (*) using real-valued FFT[Sorensen, Jones, Heaideman 1987] with 3/3 complex mult.

11 Polynomial multiplication in R[x] Polynomial Multiplication in monomial basis (theoretical count) Schoolbook method Karatsuba method DFT based method 10 6 nbr of operations FP mul = 1 FP add Polynomials size (equiv. to degree+1)

12 Polynomial multiplication in R[x] Polynomial Multiplication in monomial basis (theoretical count) Schoolbook method Karatsuba method DFT based method 10 6 nbr of operations FP mul = 2 FP add Polynomials size (equiv. to degree+1)

13 Polynomial multiplication in R[x] Polynomial Multiplication in monomial basis (theoretical count) Schoolbook method Karatsuba method DFT based method 10 6 nbr of operations FP mul = 4 FP add Polynomials size (equiv. to degree+1)

14 Monomial basis for polynomial are not the only ones!!!

15 Monomial basis for polynomial are not the only ones!!! Chebyshev basis seems to be very useful

16 In this presentation, I will assume that polynomials have degree d = n 1 where n = 2 k. formula will be using the degree d. complexity estimate will be using the number of terms n.

17 Outline 1 Polynomial multiplication in monomial basis 2 Polynomial multiplication in Chebyshev basis 3 A more direct reduction from Chebyshev to monomial basis 4 Implementations and experimentations 5 Conclusion

18 Chebyshev Polynomials: a short definition Chebyshev polynomials of the first kind on [ 1, 1] are defined as T k (x) s.t. T k (x) = cos(k arcos(x)), k N and x [ 1, 1] These polynomials are orthogonal polynomials defined by the following recurrence relation: T 0 (x) = 1 T 1 (x) = x T k (x) = 2xT k 1 (x) T k 2 (x), k > 1 Therefore, they can be used to form a base of the R-vector space of R[x].

19 Polynomials in Chebyshev basis Every a R[x] can be expressed as a linear combination of T k : a(x) = a d a k T k (x) k=1

20 Polynomials in Chebyshev basis Every a R[x] can be expressed as a linear combination of T k : a(x) = a d a k T k (x) k=1 Question: How fast can we multiply polynomials in Chebyshev basis?

21 Polynomials in Chebyshev basis Every a R[x] can be expressed as a linear combination of T k : a(x) = a d a k T k (x) k=1 Question: How fast can we multiply polynomials in Chebyshev basis? Main difficulty T i (x).t j (x) = T i+j (x)+t i j (x) 2, i, j N

22 Multiplication of polynomials in Chebyshev basis Complexity results: Let a, b R[x] two polynomials of degree d = n 1 given in Chebyshev basis, one can compute the product a.b R[x] in Chebyshev basis with O(n 2 ) op. in R [direct method] O(n log n) op. in R [Baszenski, Tasche 1997] O(M(n)) op. in R [Bostan, Salvy, Schost 2010] where M(n) is the cost of the multiplication in monomial basis.

23 Multiplication of polynomials in Chebyshev basis Complexity results: Let a, b R[x] two polynomials of degree d = n 1 given in Chebyshev basis, one can compute the product a.b R[x] in Chebyshev basis with O(n 2 ) op. in R [direct method] O(n log n) op. in R [Baszenski, Tasche 1997] O(M(n)) op. in R [Bostan, Salvy, Schost 2010] where M(n) is the cost of the multiplication in monomial basis. Remark: M(n) method is using basis conversions which increase constant term in the complexity estimate and probably introduce numerical errors.

24 Multiplication of polynomials in Chebyshev basis Complexity results: Let a, b R[x] two polynomials of degree d = n 1 given in Chebyshev basis, one can compute the product a.b R[x] in Chebyshev basis with O(n 2 ) op. in R [direct method] O(n log n) op. in R [Baszenski, Tasche 1997] O(M(n)) op. in R [Bostan, Salvy, Schost 2010] where M(n) is the cost of the multiplication in monomial basis. Remark: M(n) method is using basis conversions which increase constant term in the complexity estimate and probably introduce numerical errors. My goal is to get a more direct reduction to the monomial case!!!

25 Lets first recall existing algorithms...

26 Multiplication in Chebyshev basis: the direct method Derived from T i (x).t j (x) = T i+j (x)+t i j (x) 2, i, j N

27 Multiplication in Chebyshev basis: the direct method Derived from T i (x).t j (x) = T i+j (x)+t i j (x) 2, i, j N Let a, b R[x] of degree d, given in Chebyshev basis, then c(x) = a(x)b(x) = c 2d c k T k (x) is computed according to the following equation: d a 0 b a l b l, for k = 0, 2c k = k l=0 l=1 k=1 d k a k l b l + (a l b k+l + a k+l b l ), for k = 1,..., d 1, l=1 d a k l b l, for k = d,..., 2d. l=k d

28 Multiplication in Chebyshev basis: the direct method Derived from T i (x).t j (x) = T i+j (x)+t i j (x) 2, i, j N Let a, b R[x] of degree d, given in Chebyshev basis, then c(x) = a(x)b(x) = c 2d c k T k (x) is computed according to the following equation: d a 0 b a l b l, for k = 0, 2c k = k l=0 d l=k d l=1 k=1 d k a k l b l + (a l b k+l + a k+l b l ), for k = 1,..., d 1, l=1 a k l b l, for k = d,..., 2d. using n 2 + 2n 1 multiplications and (n 1)(3n 2) 2 additions in R.

29 Yet another quadratic method Lima, Panario and Wang [IEEE TC 2010] proposed another approach yielding a quadratic complexity which lowers down the number of multiplications n 2 + 5n 2 multiplications in R, 2 3n 2 + n log 3 6n + 2 additions in R. Main idea perform the multiplication on polynomials as if they were in monomial basis and correct the result.

30 Yet another quadratic method Lima, Panario and Wang [IEEE TC 2010] proposed another approach yielding a quadratic complexity which lowers down the number of multiplications n 2 + 5n 2 multiplications in R, 2 3n 2 + n log 3 6n + 2 additions in R. Main idea perform the multiplication on polynomials as if they were in monomial basis and correct the result. Interesting: this approach is almost halfway from a direct reduction to monomial basis case!!!

31 Yet another quadratic method [Lima, Panario, Wang 2010] Let a(x) = a d a k T k (x) and b(x) = b d b k T k (x). k=1 Compute convolutions f k = Simplifying direct method to k=1 k a k l b l using Karatsuba algorithm. l=0 d f a l b l l=1 for k = 0, d k 2c k = f k + (a l b k+l + a k+l b l ) for k = 1,..., d 1, l=1 f k for k = d,..., 2d.

32 Yet another quadratic method [Lima, Panario, Wang 2010] Let a(x) = a d a k T k (x) and b(x) = b d b k T k (x). k=1 Compute convolutions f k = Simplifying direct method to k=1 k a k l b l using Karatsuba algorithm. l=0 d f a l b l l=1 for k = 0, d k 2c k = f k + (a l b k+l + a k+l b l ) for k = 1,..., d 1, l=1 f k for k = d,..., 2d. still need to compute remaining part to get the result

33 Yet another quadratic method [Lima, Panario, Wang 2010] d f a l b l l=1 for k = 0, d k 2c k = f k + (a l b k+l + a k+l b l ) for k = 1,..., d 1, l=1 f k for k = d,..., 2d. All the terms have been already computed together in convolutions f k. could be recomputed but at an expensive cost could use separation technique to retrieve them [Lima, Panario, Wang 2010] Main idea exploit the recursive three terms structure < a 0 b 0, a 0 b 1 + a 1 b 0, a 1 b 1 > of Karatsuba s algorithm to separate all the a ik b jk + a jk b ik

34 Yet another quadratic method [Lima, Panario, Wang 2010] d f a l b l l=1 for k = 0, d k 2c k = f k + (a l b k+l + a k+l b l ) for k = 1,..., d 1, l=1 f k for k = d,..., 2d. All the terms have been already computed together in convolutions f k. could be recomputed but at an expensive cost could use separation technique to retrieve them [Lima, Panario, Wang 2010] Main idea exploit the recursive three terms structure < a 0 b 0, a 0 b 1 + a 1 b 0, a 1 b 1 > of Karatsuba s algorithm to separate all the a ik b jk + a jk b ik still needs O(n 2 ) operations and O(n log3 ) extra memory!!!

35 Fast method - DCT-based [Baszenski, Tasche 1997] Let f (x) = f 0 + f 1 x f d x d R[x], then n 1 DFT n (f ) = (F k ) k=0..n 1 such that F k = f j e 2iπ n kj j=0 n 1 DCT n (f ) = (F k ) k=0..n 1 such that F k = 2 j=0 [ ] πk f j cos (2j + 1) 2n DCT n is almost the real part of a DFT 2n of the even symmetrized input Main idea Use same approach as for monomial basis but with DCT instead of DFT.

36 Fast method - DCT-based [Baszenski, Tasche 1997] Use evaluation/interpolation on the points µ j = cos( jπ n ), j = 0,..., n 1 allows the use of DCT and its inverse. Principle Let a(x) = a n a k T k (x). k=1 Using Chebyshev polynomials definition we have which gives T k (µ j ) = cos( kπj n ) n 1 a(µ j ) = a k cos( kπj n ). k=0 This is basically a DCT n on coefficients of a(x).

37 Fast method - DCT-based [Baszenski, Tasche 1997] Let a, b R[x] given in Chebyshev basis with degree d < n = 2 k The method to compute c = ab is: set µ j = cos( jπ n ), j = 0,..., 2n [a(µ 0 ), a(µ 1 ),..., a(µ 2n )] R 2n+1 using DCT on a [b(µ 0 ), b(µ 1 ),..., b(µ 2n )] R 2n+1 using DCT on b ab = a(µ 0)b(µ 0) + a(µ 1)b(µ 1)x a(µ 2n)b(µ 2n)x 2n R[x] [c 0, c 1,..., c 2n ] R 2n using DCT on ab plus scaling

38 Fast method - DCT-based [Baszenski, Tasche 1997] Let a, b R[x] given in Chebyshev basis with degree d < n = 2 k The method to compute c = ab is: set µ j = cos( jπ n ), j = 0,..., 2n [a(µ 0 ), a(µ 1 ),..., a(µ 2n )] R 2n+1 using DCT on a [b(µ 0 ), b(µ 1 ),..., b(µ 2n )] R 2n+1 using DCT on b ab = a(µ 0)b(µ 0) + a(µ 1)b(µ 1)x a(µ 2n)b(µ 2n)x 2n R[x] [c 0, c 1,..., c 2n ] R 2n using DCT on ab plus scaling This method requires: 3n log 2n 2n + 3 multiplications in R, (9n + 3) log 2n 12n + 12 additions in R.

39 Polynomial multiplication in R[x] (Chebyshev basis) Exact number of operations in R to multiply two polynomials of R[x] with degree d < n = 2 k in Chebyshev basis Algorithm nbr of multiplication nbr of addition Direct method n 2 + 2n 1 1.5n 2 2.5n + 1 Lima et al. 0.5n n 1 3n 2 + n log3 6n + 2 DCT-based 3n log 2n 2n + 3 (9n + 3) log 2n 12n + 12

40 Polynomial multiplication in R[x] (Chebyshev basis) Polynomial Multiplication in Chebyshev basis (theoretical count) Direct method Lima et. al method DCT based method 10 6 nbr of operations FP mul = 1 FP add Polynomials size (equiv. to degree+1)

41 Polynomial multiplication in R[x] (Chebyshev basis) Polynomial Multiplication in Chebyshev basis (theoretical count) Direct method Lima et. al method DCT based method 10 6 nbr of operations FP mul = 2 FP add Polynomials size (equiv. to degree+1)

42 Polynomial multiplication in R[x] (Chebyshev basis) Polynomial Multiplication in Chebyshev basis (theoretical count) Direct method Lima et. al method DCT based method 10 6 nbr of operations FP mul = 4 FP add Polynomials size (equiv. to degree+1)

43 Outline 1 Polynomial multiplication in monomial basis 2 Polynomial multiplication in Chebyshev basis 3 A more direct reduction from Chebyshev to monomial basis 4 Implementations and experimentations 5 Conclusion

44 Use same approach as in [Lima, Panario, Wang 2010] Let a(x) = a d a k T k (x) and b(x) = b d b k T k (x). k=1 Using any polynomial multiplication algorithms (monomial basis) to compute convolutions: k=1 f k = k a k l b l l=0 d f a l b l l=1 for k = 0, d k 2c k = f k + (a l b k+l + a k+l b l ) for k = 1,..., d 1, l=1 f k for k = d,..., 2d.

45 Use same approach as in [Lima, Panario, Wang 2010] Let a(x) = a d a k T k (x) and b(x) = b d b k T k (x). k=1 Using any polynomial multiplication algorithms (monomial basis) to compute convolutions: k=1 f k = k a k l b l l=0 d f a l b l l=1 for k = 0, d k 2c k = f k + (a l b k+l + a k+l b l ) for k = 1,..., d 1, l=1 f k for k = d,..., 2d. so we do for the remaining part!!!

46 Correcting by monomial basis multiplication We need to compute: d a l b l l=1 d k a l b k+l l=1 d k a k+l b l l=1 for k = 1,..., d 1

47 Correcting by monomial basis multiplication We need to compute: d a l b l l=1 d k a l b k+l l=1 d k a k+l b l l=1 for k = 1,..., d 1 which can be deduced from convolutions d g d = r d l b l l=0 d k g d+k = r d l b k+l l=0 d k g d k = r d k l b l l=0 with an extra cost of O(n) operations, where a l = r d l

48 A direct reduction : PM-Chebyshev Let a(x), b(x) R[x] of degree d = n 1 given in Chebyshev basis and r(x) the reverse polynomial of a(x). Use any monomial basis algorithms to compute convolutions: f k = k a k l b l and g k = l=0 k r k l b l l=0

49 A direct reduction : PM-Chebyshev Let a(x), b(x) R[x] of degree d = n 1 given in Chebyshev basis and r(x) the reverse polynomial of a(x). Use any monomial basis algorithms to compute convolutions: f k = k a k l b l and g k = l=0 k r k l b l l=0 Computation of c(x) = a(x)b(x) in Chebyshev basis is deduced from : f 0 + 2(g d a 0 b 0 ) for k = 0, 2c k = f k + g d k + g d+k a 0 b k a k b 0 for k = 1,..., d 1, f k for k = d,..., 2d. at a cost of 2M(n) + O(n) operations in R.

50 Analysis of the complexity Let M(n) be the cost of polynomial multiplication in monomial basis with polynomials of degree d < n. PM-Chebyshev algorithm costs exactly 2M(n) + 8n 10 op. in R.

51 Analysis of the complexity Let M(n) be the cost of polynomial multiplication in monomial basis with polynomials of degree d < n. PM-Chebyshev algorithm costs exactly 2M(n) + 8n 10 op. in R. Optimization trick Setting a 0 = b 0 = 0 juste before computation of coefficients g k gives f 0 + 2g d for k = 0, 2c k = f k + g d k + g d+k for k = 1,..., d 1, f k for k = d,..., 2d. reducing the cost to 2M(n) + 4n 3 op. in R.

52 Analysis of the complexity Exact number of operations in R to multiply two polynomials of R[x] with degree d < n = 2 k given in Chebyshev basis M(n) nb. of multiplication nb. of addition Schoolbook 2n 2 + 2n 1 2n 2 2n Karatsuba 2n log 3 + 2n 1 14n log 3 12n + 2 DFT-based ( ) 6n log 2n 6n n log 2n 22n + 22 (*) using real-valued FFT[Sorensen, Jones, Heaideman 1987] with 3/3 complex mult.

53 Special case of DFT-based multiplication PM-Chebyshev algorithm can be degenerated to reduce the constant term It involves 2 multiplications with only 3 different operands. one DFT is computed twice

54 Special case of DFT-based multiplication PM-Chebyshev algorithm can be degenerated to reduce the constant term It involves 2 multiplications with only 3 different operands. one DFT is computed twice 1/6th of the computation can be saved giving a complexity in this case of 5n log 2n 3n + 9 multiplications in R, 15n log 2n 17n + 18 additions in R.

55 Special case of DFT-based multiplication We can trade another DFT for few linear operations. Indeed, we need to compute : DFT 2n (ā(x)) and DFT 2n ( r(x)) where ā(x) = a 0 + a 1 x a d x d and r(x) = a d + a d 1 x a 0 x d = ā(x 1 )x d. Assuming ω = e 2iπ 2n, we know that DFT 2n (ā) = [ ā(ω k ) ] k=0...2n 1, (1) DFT 2n ( r) = [ ā(ω 2n k ) ω kd ] k=0...2n 1. (2) (1) and (2) are equivalent modulo 4n 2 multiplications in C.

56 Special case of DFT-based multiplication Almost 1/3th of the computation can be saved giving a complexity in this case of 4n log 2n + 12n + 1 multiplications, 12n log 2n + 8 additions.

57 Special case of DFT-based multiplication Almost 1/3th of the computation can be saved giving a complexity in this case of 4n log 2n + 12n + 1 multiplications, 12n log 2n + 8 additions. We will now refer to our algorithm as PM-Chebyshev(XXX), where XXX represents underlying monomial basis algorithm.

58 Polynomial multiplication in Chebyshev basis (theoretical) 100 Direct method Method of [Lima et. al 2010] DCT based method PM Chebyshev (Karatsuba) PM Chebyshev (DFT based) 1 FP mul = 1 FP add Speedup Polynomials size (equiv. to degree+1)

59 Polynomial multiplication in Chebyshev basis (theoretical) 100 Direct method Method of [Lima et. al 2010] DCT based method PM Chebyshev (Karatsuba) PM Chebyshev (DFT based) 1 FP mul = 2 FP add Speedup Polynomials size (equiv. to degree+1)

60 Polynomial multiplication in Chebyshev basis (theoretical) 100 Direct method Method of [Lima et. al 2010] DCT based method PM Chebyshev (Karatsuba) PM Chebyshev (DFT based) 1 FP mul = 4 FP add Speedup Polynomials size (equiv. to degree+1)

61 Outline 1 Polynomial multiplication in monomial basis 2 Polynomial multiplication in Chebyshev basis 3 A more direct reduction from Chebyshev to monomial basis 4 Implementations and experimentations 5 Conclusion

62 Software Implementation My goal provide efficient code for multiplication in Chebyshev basis evaluate performances of existing algorithms My method use C++ for generic, easy, efficient code re-use as much as possible existing efficient code especially for DFT/DCT based code [Spiral project, FFTW library]

63 Implementing PM-Chebyshev algorithm Easy as simple calls to monomial multiplication template<c l a s s T, void mulm( vector <T>&, c o n s t vector <T>&, c o n s t vector <T>&)> v o i d mulc ( vector <T>& c, c o n s t vector <T>& a, c o n s t vector <T>& b ){ s i z e t da, db, dc, i ; da=a. s i z e ( ) ; db=b. s i z e ( ) ; dc=c. s i z e ( ) ; } v e c t o r <T> r ( db ), g ( dc ) ; f o r ( i =0; i <db ; i ++) r [ i ]=b [ db 1 i ] ; mulm( c, a, b ) ; mulm( g, a, r ) ; f o r ( i =0; i <dc;++ i ) c [ i ] = 0. 5 ; c [0]+= c2 [ da 1] a [ 0 ] b [ 0 ] ; f o r ( i =1; i <da 1; i ++) c [ i ]+= 0. 5 ( g [ da 1+i ]+g [ da 1 i ] a [ 0 ] b [ i ] a [ i ] b [ 0 ] ) ;

64 Implementation of multiplication in monomial basis Remark no standard library available for R[x] My codes naive implementations of Schoolbook and Karatsuba (recursive) highly optimized DFT-based method using FFTW library a use hermitian symmetry of DFT on real inputs a

65 Implementation of multiplication in Chebshev basis C++ based code naive implementation of direct method optimized DCT-based method of [Tasche, Baszenski 1997] using FFTW Remark No implementation of [Lima, Panario, Wang 2010] method, since needs almost as many operations as direct method requires O(n log 3 ) extra memory no explicit algorithm given, sounds quite tricky to implement

66 Implementation of multiplication in Chebshev basis C++ based code naive implementation of direct method optimized DCT-based method of [Tasche, Baszenski 1997] using FFTW Remark No implementation of [Lima, Panario, Wang 2010] method, since needs almost as many operations as direct method requires O(n log 3 ) extra memory no explicit algorithm given, sounds quite tricky to implement My feelings it would not be efficient in practice!!!

67 A note on DCT in FFTW Numerical accuracy Faster algorithms using pre/post processed read DFT suffer from instability issues. In practice, prefer to use: smaller optimized DCT-I codelet doubled size DFT According to this, our PM-Chebyshev(DFT-based) should really be competitive.

68 A note on DCT in FFTW Numerical accuracy Faster algorithms using pre/post processed read DFT suffer from instability issues. In practice, prefer to use: smaller optimized DCT-I codelet doubled size DFT According to this, our PM-Chebyshev(DFT-based) should really be competitive. But, is our code numerically correct???

69 Insight on the numerical accuracy We experimentally check the relative error of each methods. Relative error on polynomial multiplication Let a, b Fl[x] given in Chebyshev basis. Consider ĉ a.b Fl[x] and c = a.b R[x], then the relative error is E(ĉ) = c ĉ 2 c 2 1 gmplib.org

70 Insight on the numerical accuracy We experimentally check the relative error of each methods. Relative error on polynomial multiplication Let a, b Fl[x] given in Chebyshev basis. Consider ĉ a.b Fl[x] and c = a.b R[x], then the relative error is E(ĉ) = c ĉ 2 c 2 We use GMP library 1 : to get exact result as a rational rumber, to almost compute the relative error as a rational rumber. 1 gmplib.org

71 Experimental relative error Average error on random polynomials with entries lying in [ 50, 50]. 1e 12 1e 13 Direct method DCT based method DCT based (via FFT) method PM Chebyshev (Karatsuba) PM Chebyshev (DFT based) PM Chebyshev (DFT based accur) Relative error 1e 14 1e 15 1e 16 1e Polynomial size (equiv. to degree+1)

72 Experimental relative error Average error on random polynomials with entries lying in [0, 50]. 1e 14 Direct method DCT based method DCT based (via FFT) method PM Chebyshev (Karatsuba) PM Chebyshev (DFT based) PM Chebyshev (DFT based accur) 1e 15 Relative error 1e 16 1e Polynomial size (equiv. to degree+1)

73 Lets now see practical performances... based on average time estimation

74 Polynomial multiplication in Chebyshev basis: experimental performance on Intel Xeon 2GHz Direct method DCT based method DCT based (via FFT) method PM Chebyshev (Karatsuba) PM Chebyshev (DFT based) PM Chebyshev (DFT based accur) Speedup Polynomial size (equiv. to degree+1)

75 Polynomial multiplication in Chebyshev basis: experimental performance on Intel Xeon 2GHz Direct method DCT based method DCT based (via FFT) method PM Chebyshev (Karatsuba) PM Chebyshev (DFT based) PM Chebyshev (DFT based accur) 2 Speedup Polynomial size (equiv. to degree+1)

76 Polynomial multiplication in Chebyshev basis: experimental performance on Intel Xeon 2GHz DCT based method DCT based (via FFT) method PM Chebyshev (DFT based) PM Chebyshev (DFT based accur) 140 Speedup Polynomial size (equiv. to degree+1)

77 Outline 1 Polynomial multiplication in monomial basis 2 Polynomial multiplication in Chebyshev basis 3 A more direct reduction from Chebyshev to monomial basis 4 Implementations and experimentations 5 Conclusion

78 Conclusion Main contribution Provide a direct method to multiply polynomials given in Chebyshev basis which reduces to monomial basis multiplication: better complexity than using basis conversions, easy implementation offering good performances, probably as accurate as any other direct methods, offer the use of FFT instead of DCT-I.

79 Conclusion Main contribution Provide a direct method to multiply polynomials given in Chebyshev basis which reduces to monomial basis multiplication: better complexity than using basis conversions, easy implementation offering good performances, probably as accurate as any other direct methods, offer the use of FFT instead of DCT-I. Would be interesting to compare with optimized Karatsuba s implementation, further investigation on stability issues, discover similarity with finite fields (Dickson Polynomials) and see applications in cryptography [Hasan, Negre 2008].

80 Times of polynomial multiplication in Chebyshev basis (given in µs) on Intel Xeon 2GHz platform. n Direct DCT-based DCT-based (FFT) PM-Cheby (Kara) PM-Cheby (DFT) PM-Cheby (DFT accur) PM-Cheby stands for PM-Chebyshev algorithm.

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