# COMPOSITES THAT REMAIN COMPOSITE AFTER CHANGING A DIGIT

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1 COMPOSITES THAT REMAIN COMPOSITE AFTER CHANGING A DIGIT Michael Filaseta Mathematics Department University of South Carolina Columbia, SC Mark Kozek Department of Mathematics Whittier College Whittier, CA Charles Nicol Mathematics Department University of South Carolina Columbia, SC John Selfridge Dept. of Mathematical Sciences Northern Illinois University DeKalb, IL Introduction We begin with a simple example. Let m be a positive integer. Let N = ( ) m The right-most digit of N is 5. Thus, N is composite, and if we replace any digit of N, other than the right-most digit, with a different digit, then the new number obtained will be divisible by 5 and, hence, be composite. On the other hand, if we replace the right-most digit of N by a different digit, then we obtain one of N ± j where j {1, 2, 3, 4, 5} and each of these is easily seen to be composite. Thus, N has the property that if we replace any one of its digits with an arbitrary digit, the number we obtain is composite. Constructing examples of N having the above property is simplified by the fact that changing digits (arbitrarily many) of a natural number N, other than the right-most digit, results in a number divisible by gcd(n, 10). Thus, if we allow gcd(n, 10) 1, then we can easily construct examples of N with the property that replacing any one of its digits with an arbitrary digit results in a composite number Mathematics Subject Classification: 11B25 (11A07, 11A63). The first author was supported by the National Security Agency during the writing of this paper. 1

2 Our goal is to address here the more difficult issue of constructing such N but with the restriction gcd(n, 10) = 1. For example, the number N = has this property. In other words, every number in the set, { d1215, 2d215, 21d15, 212d5, 2121d, 21215d : d {0, 1, 2,..., } } is composite. In fact, it can be checked that is the smallest composite natural number, coprime to 10, such that if we replace any one of the digits of its decimal expansion with a d {0,..., }, then the number created by this replacement is composite. We prove the following: Theorem 1. There are infinitely many composite natural numbers N, coprime to 10, with the property that if we replace any digit in the decimal expansion of N with d {0,..., }, then the number created by this replacement is composite. After establishing Theorem 1, we will examine a similar problem in which the insertion of any one digit into a composite natural number N results in a number that is composite. As in the previous paragraph, we are interested in N that are coprime to 10. For example, the number N = has this property. In other words, every number in the set, { d25011, 2d5011, 25d011, 250d11, 2501d1, 25011d : d {0, 1, 2,..., } } is composite. The number is the smallest composite natural number, coprime to 10, such that if you insert any digit d {0,..., } anywhere in its decimal expansion, then the number created by this insertion is composite. Analogous to Theorem 1, we establish the following: Theorem 2. There are infinitely many composite natural numbers N, coprime to 10, with the property that if you insert any digit d {0,..., } anywhere in the decimal expansion of N, then the number created by this insertion is composite. Another problem that could be considered along similar lines is whether there are infinitely many composite numbers such that when you remove any one digit, then the number remains composite. This problem is easier, however, as can be seen by considering a number consisting of a string of 3 s. Further, a string of 1 s of the form (10 15k+10 1)/, where k is an arbitrary positive integer, provides a further slightly less obvious example. We turn to some open questions. We were able to obtain analogous results to Theorem 1 and Theorem 2 for all bases b < 12, but do analogous results hold for all bases? Do there exist infinitely many composite numbers N satisfying the conditions of both Theorem 1 and Theorem 2? Are there infinitely many primes p that are composite for every replacement (or insertion) of a digit? Does there exist a k 0 such that for every positive integer k k 0 there is a composite number N with exactly k digits having the property of Theorem 1 (or Theorem 2)? Do there exist infinitely many composite numbers N that remain composite when any two digits (not necessarily consecutive) are changed (or inserted)? Based on heuristics, we conjecture that the answers to all of the above questions except the last is in the affirmative. Acknowledgment: The first two authors note that Theorem 1 was established some years ago by the latter two authors. As it never appeared in print, the first two authors reconstructed a proof and included Theorem 2 which, as we will see, follows rather easily from the argument for Theorem 1. 2

3 2 Proof of Theorem 1 We give a construction of an infinite sequence of composite natural numbers N satisfying the conditions in the theorem. We write the decimal expansion of N as N = d n 1 d n 2... d 1 d 0, d i {0,..., }, n 1, d n 1 0. We also express N in the form N = 10n 1 + M, where M is a fixed natural number to be determined and n is a large natural number. Observe that our last expression for N is equivalent to N = } 11.{{.. 11} +M n-many 1 s where the number is a string of n digits that are 1. To establish the theorem, we will find an infinite arithmetic progression of natural numbers n, a fixed natural number M, and a finite set of primes P, such that when we replace any digit in the decimal expansion of N with x {0,..., }, then the number created by this replacement is divisible by at least one of the primes in P. We suppose throughout that n is large enough to imply that the left-most digit of N is 1. Let N (k) (x) be the number that is obtained by replacing the digit d k of the decimal expansion of N with x {0,..., }. For example, if N = = d 5 d 4 d 3 d 2 d 1 d 0, then N (2) (3) = Since M is fixed, there exists a non-negative integer K n 1 such that d k = 1 for all k K. We first consider these large values of k. For these k, we have that N (k) (x) takes the form Observe that if N (k) (x) = 10n 1 then N (k) (x) is divisible by 3 since, in this case, N (k) (x) = 10n 1 + M + (x 1) 10 k. n 0 (mod 3) M 1 (mod 3) x 0 (mod 3), + M + (x 1) 10 k ( 1) (mod 3). Thus, the conditions n 0 (mod 3) and M 1 (mod 3) imply that for large k if we replace d k with x {0, 3, 6, }, then we obtain a composite number N (k) (x). Similarly, we observe that if n 0 (mod 6) M 0 (mod 7) x 1 (mod 7), 3

4 then N (k) (x) is divisible by 7. Thus, for large k we know that our original N is necessarily composite and that if we replace d k by x = 8, then we obtain the composite number N (k) (x). Combining the above, we see that if n 0 (mod 6) and M 7 (mod 21) and if, for an arbitrary k K, we replace d k in N with any digit in {0, 1, 3, 6, 8, }, then the number N (k) (x) obtained is divisible by either 3 or 7. To address the remaining cases of N (k) (x) for x {2, 4, 5, 7} when k is large, we will use the following. Definition. A finite system of congruences x a i (mod m i ), 1 i t, is called a covering of the integers if each integer satisfies at least one congruence in the system. As an example, we note x 0 (mod 2) x 1 (mod 3) x 3 (mod 4) x 5 (mod 6) x (mod 12) is a covering of the integers. In the way of notation, for a prime p and an integer a not divisible by p, we use ord p (a) to denote the order of a modulo p, that is the least positive integer k such that a k 1 (mod p). Lemma 1. Let N and M be natural numbers such that where N has the decimal expansion N = 10n 1 + M, N = d n 1 d n 2... d 1 d 0, d i {0,..., }, n 1, d n 1 0. Let K be a non-negative integer n 1 such that d k = 1 for k {K, K + 1,..., n 1}. For a fixed x {0,... }, suppose we have distinct primes p 1,..., p t, each > 5, for which (i) there exists a covering of the integers where c i = ord pi (10), (ii) n 0 (mod lcm(c 1,..., c t )), (iii) M is a solution to the system of congruences k b i (mod c i ), 1 i t, M (x 1) 10 b i (mod p i ), 1 i t. 4

5 Then, for each k {K, K + 1,..., n 1}, N (k) (x) = 10n 1 + M + (x 1) 10 k is divisible by at least one of the primes p i where 1 i t. Proof. Suppose the conditions in the lemma hold. Let k {K, K+1,..., n 1}. By (i), there is an i {1,..., t} such that k b i (mod c i ). Since c i = ord pi (10) and n 0 (mod lcm(c 1,..., c t )), we have p i (10 n 1)/. Hence, N (k) (x) = 10n 1 + M + (x 1) 10 k M + (x 1) 10 b i (mod p i ). From (iii), we deduce N (k) (x) 0 (mod p i ), completing the proof. Set P 0 = P 3 = P 6 = P = {3}, P 1 = P 8 = {7}. For x {2, 4, 5, 7}, let P x denote a set of primes, if it exists, as in Lemma 1 so that N (k) (x) is divisible by some prime from P x for each k {K, K + 1,..., n 1} provided (ii) and (iii) hold for some covering as in (i). For primes p {2, 5}, we define c(p) = ord p (10). For x {2, 4, 5, 7}, we write the covering system in (i) as k b(x, p) (mod c(p)), p P x. (1) To establish Theorem 1, we will take n in the definition of N so that it is divisible by the least common multiple of the numbers in the set {c(p) : p P x }. x=0 We will also take M so that the various congruences M 7 (mod 21) M (x 1) 10 b(x,p) (mod p), where x {2, 4, 5, 7} and p P x, (2) simultaneously hold. It is clear that we can find infinitely many n as above, but we need our choice of coverings as in (1) and prime sets P x to be such that (2) has a solution. We justify that this is possible with explicit sets P x and coverings as in (1). To obtain a set of primes P x and a covering as in (1), we make use of the tables of factorizations of 10 c 1 in [1] to determine, for a given positive integer c, the set of primes p such that c = ord p (10). For the purposes of finding a covering as in (1), each such p corresponds to a congruence of the form k b (mod c) that we can use, where we have some freedom on how to choose b. Observe that, given c, there may be more than one prime p for which c = ord p (10). Choosing b differently for different p allows us to use two or more different congruences with the same modulus in a covering. On the other hand, given c, it is also possible that there are no primes p for which c = ord p (10). In this case, we cannot use the modulus c in (1). 5

6 To clarify, one can check that 2 = ord 11 (10) 4 = ord 101 (10) 8 = ord 73 (10) 8 = ord 137 (10). Thus, for a covering of the integers, we may use 2 and 4 each once as the modulus of a congruence of a covering, and we may use 8 twice as the modulus of a congruence. One checks that k 0 (mod 2) k 1 (mod 4) k 3 (mod 8) k 7 (mod 8) is a covering of the integers. To apply Lemma 1, we take n divisible by 8, and M so that P 2 = {11, 73, 101, 137}, M (2 1) 10 0 (mod 11) M (2 1) 10 1 (mod 101) M (2 1) 10 3 (mod 73) M (2 1) 10 7 (mod 137). Note that these congruences on M are equivalent to a single congruence modulo by the Chinese Remainder Theorem; in particular, such M exist. We deduce from Lemma 1 that, under these conditions, the number N (k) (2) is composite for K k n 1. We write the congruence on M just alluded to above as M B(2) (mod ). Combining this with our previous results, we have that if n 0 (mod 24) and M is a solution of the system of congruences M 7 (mod 3 7) M B(2) (mod ), which we know exists by the Chinese Remainder Theorem, then N (k) (x) will be divisible by at least one prime in {3, 7, 11, 73, 101, 137} for each x {0, 1, 2, 3, 6, 8, } and K k n 1. 6

7 Observe that to assure that M exists by using the Chinese Remainder Theorem above, we chose P 2 so that it had an empty intersection with the previous sets P 0, P 1, P 3, P 6, P 8, P already constructed. This will be our strategy then for establishing that the congruences in (2) simultaneously hold. Thus, we follow the same approach for N (k) (x) when x {4, 5, 7} as we did for N (k) (2), but, in each case, the covering of the integers will be more complicated because we do not wish to repeat using the same primes previously chosen. We list the coverings and the primes used for x {4, 5, 7}. We begin with x = 4. Table 1 contains the needed information for us. To conserve space, we note here that p 7 = , p 14 = row congruence prime p i 1 k 0 (mod 3) 37 2 k 1 (mod 6) 13 3 k 2 (mod ) k 5 (mod 18) 1 5 k 14 (mod 18) k 8 (mod 27) k 17 (mod 27) p 7 row congruence prime p i 8 k 26 (mod 54) k 53 (mod 54) k 4 (mod 12) k 10 (mod 24) k 22 (mod 72) k 46 (mod 72) k 70 (mod 72) p 14 Table 1: Covering used in Lemma 1 (i) for N (k) (4) We observe that we are taking P 4 to be the set of 14 primes appearing in the columns with the heading prime p i. The congruences in (1) appear in the columns with heading congruence. Thus, if p appears in the column with heading prime p i, then the congruence in this same row takes the form k b(4, p) (mod c(p)). In each row, one can check directly that ord p (10) = c(p). We also need to justify that the 14 congruences displayed in Table 1 form a covering. The least common multiple of the moduli appearing in these congruences is 216. One checks directly (by hand or computer) that the numbers 0, 1, 2,..., 215 each satisfy at least one of the congruences in the table. Now, suppose that m is an arbitrary integer. Let m {0, 1,..., 215} such that m m (mod 216). We deduce then, since each modulus in the congruences divides 216, that m is a solution of whatever congruence m satisfies. Hence, the verification that the congruences in Table 1 form a covering is complete. For x {5, 7}, we use the same approach as above. We give the tables explicitly but do not elaborate on the details. We simply note that in each case one should check that the congruence k b(x, p) (mod c(p)) and p appearing on a row are such that ord p (10) = c(p). For each table, one should also check that the congruences form a covering, and this can be done by simply checking if the non-negative integers up to one less that the least common multiple of the moduli each satisfy at least one congruence from the table. For x = 5, we make use of the information in Table 2. To conserve space, we set p 10 = , p 17 =

8 row congruence prime p i 1 k 0 (mod 5) 41 2 k 1 (mod 5) k 2 (mod 10) 01 4 k 3 (mod 20) k 13 (mod 20) k 7 (mod 30) k 17 (mod 30) k 27 (mod 30) 2161 k 8 (mod 40) k 28 (mod 40) p 10 row congruence prime p i 11 k 18 (mod 60) k 38 (mod 60) k 58 (mod 60) k 4 (mod 15) k (mod 15) k 14 (mod 45) k 2 (mod 45) p k 44 (mod 0) k 8 (mod 0) Table 2: Covering used in Lemma 1 (i) for N (k) (5) For x = 7, we use the information in Table 3. We note here that p 11 = , p 13 = , p 14 = , p 15 = , p 16 = , p 21 = , p 27 = , p 31 = , p 32 = , p 34 = , p 35 = , p 36 = , p 37 = For K k n 1, our argument is complete upon noting that the sets {3, 7}, P 2, P 4, P 5 and P 7 are pairwise disjoint. This allows us then to take n so that it is divisible by 6 and each modulus appearing in (1) independent of the value of x {2, 4, 5, 7} and also assures that there is a simultaneous solution to the congruences appearing in (2). We fix C to be the least common multiple of the moduli appearing in the congruences in (1) for x {2, 4, 5, 7}. Note that 6 divides C. Thus, taking P = {3, 7} P 2 P 4 P 5 P 7, we see that if n 0 (mod C), K k n 1 and x {0, 1,..., }, then N (k) (x) is divisible by at least one prime from P. It is worth noting here that the primes 2 and 5 are not in P. This is done deliberately as Theorem 1 requires that N be coprime to 10. Combining the congruences appearing in (2) with the congruence M 0 (mod 10), for example, allows us to deduce that we can in fact take N coprime to 10. For the remainder of this section, we fix M 0 (mod 10) so that N is coprime to 10 and if n 0 (mod C), K k n 1 and x {0, 1,..., }, then N (k) (x) is divisible by at least one prime from P. Fix also some n 0 0 (mod C). We suppose that n 0 satisfies 10 n0 2 > M. Then for n n 0, the number N = 10n 1 + M 8

9 row congruence prime p i 1 k 0 (mod 7) 23 2 k 1 (mod 7) k 2 (mod 21) 43 4 k (mod 21) k 16 (mod 21) k 3 (mod 14) k 10 (mod 28) 2 8 k 24 (mod 28) 281 k 4 (mod 35) k 11 (mod 35) k 18 (mod 35) p k 25 (mod 70) k 60 (mod 70) p k 32 (mod 105) p k 67 (mod 105) p k 102 (mod 105) p k 5 (mod 42) k 26 (mod 42) k 12 (mod 42) 4561 row congruence prime p i 20 k 33 (mod 84) k 75 (mod 84) p k 1 (mod 63) k 40 (mod 63) k 61 (mod 63) k 6 (mod 28) k 13 (mod 56) k 41 (mod 56) p k 20 (mod 140) k 48 (mod 140) k 76 (mod 140) k 104 (mod 140) p k 132 (mod 140) p k 27 (mod 112) k 83 (mod 112) p k 55 (mod 168) p k 111 (mod 168) p k 167 (mod 168) p 37 Table 3: Covering used in Lemma 1 (i) for N (k) (7) has left-most digit 1. More importantly, as M is fixed, the number K is well-defined and does not vary as n varies over the integers n 0. There are a finitely many ways, independent of n n 0, to have k {0, 1,..., K 1} and x {0, 1,..., }. Fix N 0 = 10n M. Observe that N 0 is coprime to 10. Analogous to our previous notation, we refer to N (k) 0 (x) as the number obtained by replacing the digit x {0, 1,..., } for the digit of N 0 appearing in the (k + 1)st position on the right. For each k {0, 1,..., K 1} and x {0, 1,..., }, we consider the least prime q = q(k, x) dividing N (k) 0 (x). If q > 5, n n 0 (mod c(q)) and n n 0, then q divides N (k) (x) since N (k) (x) N (k) 0 (x) = 10n 10 n 0 = 10n 0 ( 10 n n 0 1 ). If q {2, 5}, then N 0 being coprime to 10 implies k = 0. It follows in this case that N (k) (x) = N (0) (x) is also divisible by q. The definition of n 0 implies that if q = 3 and n n 0 (mod 3), then both n and n 0 are divisible by 3 so that 10 n n 0 1 is divisible by 27. Thus, in this case, we also have that N (k) (x) is divisible by q. Set C to be the least common multiple of C and the numbers c(q) where q varies over the

10 primes q(k, x) > 5 with 0 k K 1 and x {0, 1,..., }. Then we deduce that if where N = 10n 1 + M, n n 0 (mod C ), then N (k) (x) is divisible by the prime q(k, x) provided 0 k K 1 and x {0, 1,..., }. As such n are necessarily divisible by C, we further obtain for such n that N (k) (x) is divisible by at least one of the primes in P for K k n 1 and x {0, 1,..., }. Since the number N tends to infinity with n and the number of primes q(k, x) as well as the set P are finite, the theorem follows. 3 Proof of Theorem 2 For a number N = d r d r 1... d 0 written in base 10 with d r 0 and digit x {0, 1,..., }, we denote by N (k) (x) the number obtained by inserting the digit x between d k and d k 1. For example, N (2) (x) = d r d r 1... d 3 d 2 xd 1 d 0. We also define N (0) (x) as the number obtained by inserting in N the digit x to the right of d 0 and N (r+1) (x) as the number obtained by inserting in N the digit x to the left of d r. Thus, N (0) (x) = d r d r 1... d 1 d 0 x and N (r+1) (x) = xd r d r 1... d 1 d 0. We fix P to be the set of primes from the proof of Theorem 1 and C, M and K as determined there so that if n 0 (mod C), then the number N = 10n 1 has the property that, for K k n 1, replacing the digit 1 that appears as the (k + 1)st digit from the right with x {0, 1,..., } results in a number N (k) (x) that is divisible by a prime in P. Setting N = 10n M, we see that N (k) (x) = N (k) (x) for K k n 1. Observe that, in the notation of the previous paragraph, r = n 2. Also, N has the property that if n 0 (mod C), then the insertion N (k) (x), for x {0, 1,..., } and K k n 1, is divisible by some prime from the set P. Note that the exponent n 1 appearing in the definition of N above is 1 modulo C. The rest of our argument is analogous to what we did at the end of the previous section. We fix M so that N = 10n 1 + M 10 + M

11 is coprime to 10 and if n 1 (mod C), K k n and x {0, 1,..., }, then N (k) (x) is divisible by at least one prime from P. Fix also some n 0 1 (mod C). We suppose that n 0 satisfies 10 n0 2 > M. Then for n n 0, the number N has left-most digit 1. As M is fixed, the number K is well-defined and does not vary as n varies over the integers n 0. There are a finitely many ways, independent of n n 0, to have k {0, 1,..., K 1} and x {0, 1,..., }. Fix N 0 = 10n M. Observe that N 0 is coprime to 10. We refer to N 0(k) (x) as the number obtained by inserting the digit x {0, 1,..., } to the left of the kth digit of N 0. For each k {0, 1,..., K 1} and x {0, 1,..., }, we consider the least prime q = q(k, x) dividing N 0(k) (x). Recall the definition c(p) = ord p (10), for p a prime, used in the previous section. Observe that if q > 5, n n 0 (mod c(q)) and n n 0, then q divides N (k) (x) since N (k) (x) N 0 (k) 10 n+1 10 n 0+1 (x) = = 10n 0+1 ( 10 n n 0 1 ). If q {2, 5}, then N 0 being coprime to 10 implies k = 0. It follows in this case that N (k) (x) = N (0) (x) is also divisible by q. If q = 3 and n n 0 (mod 3), then n n 0 is divisible by 3 so that 10 n n 0 1 is divisible by 27. Thus, in this case, we also have that N (k) (x) is divisible by q. Set C to be the least common multiple of C and the numbers c(q) where q varies over the primes q(k, x) > 5 with 0 k K 1 and x {0, 1,..., }. Then we deduce that if where N = 10n 1 + M, n n 0 (mod C ), then N (k) (x) is divisible by the prime q(k, x) provided 0 k K 1 and x {0, 1,..., }. As such n are necessarily 1 modulo C, we further obtain for such n that N (k) (x) is divisible by at least one of the primes in P for K k n and x {0, 1,..., }. Since the number N tends to infinity with n and the number of primes q(k, x) as well as the set P are finite, the theorem follows. Reference [1] J. Brillhart, D. H. Lehmer, J. L. Selfridge, B. Tuckerman, and S. S. Wagstaff, Jr., Factorizations of b n ± 1, b = 2, 3, 5, 6, 7, 10, 11, 12 Up to High Powers, 3rd edition, Contemporary Mathematics, Vol. 22, American Math. Soc., Providence, 2002 (available online). 11

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