# A characterization of trace zero symmetric nonnegative 5x5 matrices

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1 A characterization of trace zero symmetric nonnegative 5x5 matrices Oren Spector June 1, 009 Abstract The problem of determining necessary and sufficient conditions for a set of real numbers to be the eigenvalues of a symmetric nonnegative matrix is called the symmetric nonnegative inverse eigenvalue problem (SNIEP). In this paper we solve SNIEP in the case of trace zero symmetric nonnegative 5 5 matrices. 1. Introduction The problem of determining necessary and sufficient conditions for a set of complex numbers to be the eigenvalues of a nonnegative matrix is called the nonnegative inverse eigenvalue problem (NIEP). The problem of determining necessary and sufficient conditions for a set of real numbers to be the eigenvalues of a nonnegative matrix is called the real nonnegative inverse eigenvalue problem (RNIEP). The problem of determining necessary and sufficient conditions for a set of real numbers to be the eigenvalues of a symmetric nonnegative matrix is called the symmetric nonnegative inverse eigenvalue problem (SNIEP). All three problems are currently unsolved in the general case. Loewy and London [6] have solved NIEP in the case of matrices and RNIEP in the case of matrices. Moreover, RNIEP and SNIEP are the same in the case of n n matrices for n. This can be seen from papers by Fielder [] and Loewy and London [6]. However, it has been shown by Johnson et al. [] that RNIEP and SNIEP are different in general. More results about the general NIEP, RNIEP and SNIEP can be found in [1]. Other results about SNIEP in the case n = 5 can be found in [7] and [8]. Reams [9] has solved NIEP in the case of trace zero nonnegative matrices. Laffey and Meehan [] have solved NIEP in the case of trace zero nonnegative 5 5 matrices. In this paper we solve SNIEP in the case of trace zero symmetric nonnegative 5 5 matrices. 1

2 The paper is organized as follows: In Section we present some notations. We also give some basic necessary conditions for a spectrum to be realized by a trace zero symmetric nonnegative 5 5 matrix. In Section we state our main result without proof. In Section we present some preliminary results that are needed for the proof of the main results. Finally, in Section 5 we prove our main result.. Notations and some necessary conditions Let R be the set of trace zero nonnegative 5 5 matrices with real eigenvalues. Let A R. We call a spectrum σ = σ A a normalized spectrum if σ = λ 1, λ, λ, λ, λ 5 5 with 1 = λ 1 λ λ λ λ 5 1. Since A has a zero trace λ i = 0 i=1. Let R be the set of trace zero symmetric nonnegative 5 5 matrices. Then R R. Notice that for a normalized spectrum σ = λ 1, λ, λ, λ, λ 5 : λ 5 = 1 λ 5 + λ 5 + λ 5 + λ 5 1 λ + λ + λ + λ 5 = 1 λ 1 = 1, λ + λ + λ = λ 1 λ 5 = 1 λ =, λ + λ + λ = λ 1 λ = 0. Let d = λ + λ + λ. Then d, 0, λ = d λ λ and λ 5 = d 1. For the rest of this paper we shall use x instead of λ and y instead of λ. We shall deal mainly with spectra of the form σ = 1, x, y, d x y, d 1. We are interested in finding necessary and sufficient conditions for σ = 1, x, y, d x y, d 1 to be a normalized spectrum of a matrix A R. We start by looking at the necessary conditions that a normalized spectrum σ = 1, x, y, d x y, d 1 imposes. From 1 x y d x y d 1 we get: x 1, y x, y x + d + 1, y 1 d x.

3 Let the following points on the xy plain be defined: A = 1 d, 1 d, B = d + 1, d + 1, C = d +, d 1, D = 1,d, E = 1, 1 d 1, F = d + 1, d, G = d + 1, 1, H = f d, f d, I = d g d, d + 1 g(d), J = d + 1,0, O = 0,0, where, f d = d d r(d) r(d), r d = d + 7d + 7d + d d + 1 8d + 7d + 7, g d = d + 5 d + 1 d 5. A few notes are in order: 1. r(d) is real for d 1 so f(d) is real for d 1 (except perhaps when d = 0),. g(d) is real for d + 5.

4 For d, 1 the above normalization conditions form the triangle ABC in the xy plain. Note that for d = the triangle becomes a single point with coordinates 1, 1. For d 1, 0 the conditions form the quadrangle ABDE in the xy plain. Another necessary condition we shall use is due to McDonald and Neumann [8] (Lemma.1). Theorem 1 (MN): Let A be a 5 5 irreducible nonnegative symmetric matrix with a spectrum σ(a) = λ 1, λ, λ, λ, λ 5 such that λ 1 λ λ λ λ 5. Then λ + λ 5 trace(a). Loewy and McDonald [7] extended this result to any 5 5 nonnegative symmetric matrix (not just irreducible). In our case σ = 1, x, y, d x y, d 1 so we get the necessary condition x d + 1. The MN condition is already met when d, 1, while it limits the realizable set to the quadrangle ABFG when d 1, 0. For d + 5, 0 let P be the 5-vertex shape AHIFG such that all its edges but HI are straight lines and the edge HI is described by the curve γ: [0,1] R, where γ t = x(t), x(t), x t = 1 t f d + t d g d, t = 1 t d + 1 t + dt d t d d d. t d We shall show later that t is real for t f d, d g d, f(d) = f(d) and d g d = well defined. d + 1 g d, so that γ 0 = H, γ 1 = I which makes P

5 Let r d = d + 7d + 7d d d + 1 8d + 7d + 7. For d < 0 r d = d d + 7d d + 1 8d + 7d + 7, r d = r d r d r d = 16d 5 d + 7d d + 1 8d + 7d + 7. Note that lim r(d) d 0 = 16 which means r d = d 5 5 O(d5 ), so lim d 0 f(d) = 0. Moreover, g 0 = 1 and when d = 0 we have t = 0 for t 0. Therefore, when d = 0 we get H = O = A and I = J = F = D so P becomes the triangle AFG = OJG. See Appendix A for the orientation of the points A J,O in the plain for different values of d.. Statement of main result The following theorem completely solves SNIEP in the case of trace zero symmetric nonnegative 5 5 matrices. Theorem (Main result): A necessary and sufficient condition for σ = 1, x, y, d x y, d 1 to be a normalized spectrum of a matrix A R is: a) (x, y) lies within the triangle ABC for d, 1, b) (x, y) lies within the quadrangle ABFG for d 1, + 5, c) (x, y) lies within P for d + 5, 0. An immediate corollary of Theorem is: Theorem : Let σ = λ 1, λ, λ, λ, λ 5 and let s k = 5 k i=1 λ i. Suppose λ 1 λ λ λ λ 5. Then σ is a spectrum of a matrix A R if and only if the following conditions hold: 1. s 1 = 0,. s 0,. λ + λ Preliminary results In order to prove Theorem we need several results. 5

6 The first result is due to Fiedler [], which extended a result due to Suleimanova [10]: Theorem (Fiedler): Let λ 1 0 λ λ n and i=1 λ i 0. Then there exists a symmetric nonnegative n n matrix A with a spectrum σ = λ 1, λ,, λ n, where λ 1 is its Perron eigenvalue. n The second result is due to Loewy [5]. Since this result is unpublished we shall give Loewy's proof. Theorem 5 (Loewy): Let n, λ 1 λ 0 λ λ n, i=1 λ i 0. Suppose K 1, K is a partition of,,, n such that λ 1 i K1 λ i i K λ i. Then there exists a nonnegative symmetric n n matrix A with a spectrum σ = λ 1, λ,, λ n, where λ 1 is its Perron eigenvalue. n For the proof of Theorem 5 we shall need another result of Fiedler []: Theorem 6 (Fiedler): Let the following conditions hold: 1. α 1, α,, α m is realizable by a nonnegative symmetric m m matrix with α 1 as its Perron eigenvalue,. β 1, β,, β n is realizable by a nonnegative symmetric n n matrix with β 1 as its Perron eigenvalue,. α 1 β 1,. ε 0. Then α 1 + ε, β 1 ε, α,, α m, β,, β n is realizable by a nonnegative symmetric m + n (m + n) matrix with α 1 + ε as its Perron eigenvalue. Proof of Theorem 5 (Loewy): Let ε = λ 1 + ε 0. We deal with two cases. i K1 λ i. Then by the assumptions If λ 1 ε λ + ε then by the assumptions the set λ 1 ε λ i i K 1 meets the conditions of Theorem. Thus, there exists a symmetric nonnegative matrix whose eigenvalues are λ 1 ε λ i i K 1 with λ 1 ε as its Perron eigenvalue. Similarly, the set λ + ε λ i i K meets the conditions of Theorem, so there exists a symmetric nonnegative matrix whose eigenvalues are λ + ε λ i i K, where λ + ε is its Perron eigenvalue. As λ 1 ε λ + ε and ε 0 all the conditions of Theorem 6 are met and therefore the set λ 1, λ,, λ n is realizable by a nonnegative symmetric n n matrix with λ 1 as its Perron eigenvalue. 6

7 If λ 1 ε < λ + ε then let δ = 1 λ 1 λ 0. We have ε > 1 λ 1 λ = δ and λ 1 δ = λ + δ = 1 λ 1 + λ 0. Also λ 1 δ + i K1 λ i = ε δ > 0, so the set λ 1 δ λ i i K 1 meets the conditions of Theorem. We infer that there exists a symmetric nonnegative matrix whose eigenvalues are λ 1 δ λ i i K 1 with λ 1 δ as its Perron eigenvalue. Also, λ + δ + λ i i K λ + δ + λ i i K 1 = λ + δ + ε λ 1 = δ + ε δ = ε δ > 0. Therefore, the set λ + δ λ i i K meets the conditions of Theorem. Thus, there exists a symmetric nonnegative matrix whose eigenvalues are λ + δ λ i i K with λ + δ as its Perron eigenvalue. Again, all the conditions of Theorem 6 are met and therefore the set λ 1, λ,, λ n is realizable by a nonnegative symmetric n n matrix with λ 1 as its Perron eigenvalue. This completes the proof of the theorem. Let d 1, 0 and let σ = λ 1, λ, λ, λ, λ 5. Define s k = 5 k i=1 λ i. A necessary condition for σ to be a spectrum of a nonnegative 5 5 matrix A is s k 0 for k = 1,,,. This easily follows from the fact that s k = trace(a k ). We shall investigate the properties of s k for σ = 1, x, y, d x y, d 1 when (x, y) lies within the triangle OBJ. Note that for d 1, 0 the triangle OBJ is contained in the triangle ABC. Lemma 1: Let d 1, 0 and a positive integer k be set. Let s k = 1 + x k + y k + d x y k + d 1 k where (x, y) lies within the triangle OBJ. The following table summarizes the minimum and maximum values of s k over the triangle OBJ: k Even Odd Minimum achieved at 0,0 d + 1, d + 1 Minimum value 1 + d k + d 1 k 1 + d + 1 k + d 1 k Maximum achieved at d + 1,0 0,0 Maximum value 1 + (d + 1) k + d 1 k 1 + d k + d 1 k 7

8 Proof: First we note that for d = 1 the triangle OBJ becomes a single point with coordinates (0,0). In this case s k = k, so for even k we have k 1 k k s k = = = 1 + ( 1 + 1)k k, and for odd k we have k 1 k k s k = 1 1 = = k + 1 k 1. The triangle OBJ is characterized by the following inequalities: y x, y x + d + 1, y 0. We investigate s k within the triangle by looking at lines of the form y = wx, where w 0,1. These lines cover the entire triangle. For a given w we have x 0, d+1 w+1. s k = 1 + x k + y k + d x y k + d 1 k = 1 + w k + 1 x k + d w + 1 x k + d 1 k, s k x = k(wk + 1)x k 1 k w + 1 d w + 1 x k 1. Consider d = 0. As x 0 and w 0, for even k we have s k x = k wk w + 1 k x k 1 0, and for odd k we have s k x = k wk + 1 w + 1 k x k

9 Next assume 1 d < 0. The derivative is zero when k(w k + 1)x k 1 = k w + 1 d w + 1 x k 1. We know d w + 1 x 0. Otherwise, we must have x = 0 and therefore d = 0, which is a contradiction to our assumption. Then, as 0 w 1, we have x d (w + 1)x k 1 = w + 1 w k Define 1 w, k = k 1 w + 1 w k Then, for every k the derivative is zero at x = d 1 (w, k) w w, k For odd k the derivative is zero also at x = d 1 (w, k) w w, k 1 0. In the triangle OBJ we have x y 0, so the derivative of s k has the same sign for x 0, d+1. The derivative at x = 0 equals k(w + w+1 1)dk 1. Therefore, for odd (even) k the derivative is negative (positive). Thus, for d 1, 0 and w 0,1 we proved the following: For odd (even) k the maximum (minimum) value of s k over the line y = wx is achieved at x = 0 and the minimum (maximum) value of s k over the line y = wx is achieved at x = d+1. w+1 At x = 0 we have s k = 1 + d k + d 1 k and at x = d+1 w+1 we have s k = (w, d, k), where w, d, k = 1 + w k + 1 d + 1 w + 1 k + d 1 k. The derivative of with respect to w is w = k w k k wk 1 w + 1 k+1 w + 1 k d + 1 k. Since we already dealt with the case d = 1 we can assume d 1. Therefore, the derivative is zero when 9

10 k wk 1 w + 1 k = k w k + 1 w + 1 k+1, w + 1 w k 1 = w k + 1, w k 1 = 1. So, for every k the derivative is zero at w = 1. For odd k the derivative is zero also at w = 1. In any case, the derivative has the same sign in the range 1,1 of w. Since we assume 1 < d 0 at w = 0 the derivative is negative and its value is k d + 1 k. Thus, the minimum of is achieved at w = 1 and its value there is 1 + (d + 1 )k + d 1 k. The maximum of is achieved at w = 0 and its value there is 1 + (d + 1) k + d 1 k. This completes the proof of the lemma. Lemma : Let s = 1 + x + y + d x y + d 1. Then, 1. If d 1, + 5 then s 0 within the triangle OBJ,. If d + 5, 0 then s 0 within the -vertex shape OHIJ, which is formed by the intersection of the shape P with the triangle OBJ. Proof: Note that s = 1 + x + y + d x y + d 1 = d x y + dx d x y + dx d x d d. By Lemma 1 the minimum value over the triangle OBJ is 1 + d d 1 = d 9 d = d + 5 d 5. Therefore, for d 1, + 5 we have s 0 over the entire triangle OBJ. For d + 5, 0 we shall find when s 0. When d = 0 we get s = xy(x + y). As x y 0 in the triangle we conclude that s 0 if and only if y = 0. We already know that the when d = 0 the shape P 10

11 becomes the triangle OJG, so the intersection with the triangle OBJ is the line OJ. Therefore, the lemma is proved in this case. Assume that d < 0. Then x 0 > d so s is a quadratic function in the variable y. The roots of s as a function of y are: y 1 = 1 x d + 1 x + dx d x d d d, x d y = 1 x d 1 x + dx d x d d d. x d Note that y 1 is exactly the function t defined at the beginning of this paper with t replaced by x. First we find when y 1, y are real. As x > d we require x = x + dx d x d d d 0. The derivative of with respect to x is x = x + dx d = x + d x 1 d. So has a local minimum at x = d and a local maximum at x = 1 d. Since d < 0 we get 1 d = 7 d d d = d( 8 7 d + d + 1) 0, d = d d = d(d + 1) 0. Therefore, has a single real root x (d) 1 d < 0. For x x (d) we have x 0. By using the formula for the roots of a cubic equation we get x d = 1 d + d + (d) where, (d), d = d + 7d + 7d + d d + 1 8d + 7d

12 Note that d < 0 for 1 d < 0. Otherwise, x d 1 d > 0 > 1 d x d, which is a contradiction. Also note that d is exactly the function r d defined at the beginning of this paper. We conclude that y 1, y are real for x x (d). In that case y 1 y. As x > d we have y 0 and when y is in the range y, y 1 we have s 0. Next we check for what values of x the function y 1 intersects the triangle OBJ. Notice that: x = (x d) + d x + 1 x d 1. Inside the triangle OBJ we have 0 x d + 1. As d < 0 we get x d 1 d < 0. Therefore, x (x d) 0. We conclude that y 1 0. We first check when y 1 x: x 1 x d + 1 x + dx d x d d d, x d x 1 d x + dx d x d d d, (x d) x 1 d x d x + dx d x d d d 0, 8x 16dx + 8d x + d + d 0. Let 1 x = 8x 16dx + 8d x + d + d. The derivative of with respect to x is x = x dx + 8d = x d x 1 d. At x = d there is a local maximum of and its value there is d = d + d = d d + 1 < 0. Therefore, has a single real root. The derivative is positive for x > 1 d. Also, as + 5 d < 0, 0 = d + d = d d + 1 < 0,

13 d + 1 = d + 6d + 1 = d + 5 d 5 0. We conclude that there is a single root of in the range 0, d + 1, which we denote by x d. By using the formula for the roots of a cubic equation we get x d = d d d d, where d is as before. Also note that x d is exactly the function f d defined at the beginning of this paper. We found that y 1 x for + 5 d < 0 and x x d, d + 1. In other words, for x f d, d + 1 the only pairs x, y that lie in the triangle and meet the condition s 0 are those that have 0 y (x). In particular, as we stated at the beginning of this paper: f d = y 1 x d = x d = f d. Next we check when y 1 x + d + 1: x + d x d + 1 x + dx d x d d d, x d 1 + d 1 x x + dx d x d d d, (x d) 1 + d 1 x x d x + dx d x d d d 0, (d + 1)x + (d + 1) x 8d (d + 1) 0. Let 5 x = (d + 1)x + (d + 1) x 8d d + 1. Note that 5 is a quadratic since we assume d + 5 > 1. The roots of 5 are: 1 p 1 d = d d + 5 d + 1 d 5,

14 p d = d + 1 d + 5 d + 1 d 5. Note that p 1 d = d g(d), where g d is the function defined at the beginning of this paper. These roots are real since we deal with the case d + 5. Also, p 1 d d + 1 p (d), and 5 x 0 if and only if x p (d), p 1 (d). Finally, since 5 d + 1 = 8d d + 1 < 0 and d + 1 d + 1 we get that p 1 (d) d + 1. We found that y 1 x + d + 1 for + 5 d < 0 and x d + 1, p 1(d). In other words, for x d + 1, d g(d) the only pairs x, y that lie in the triangle and meet the condition s 0 are those that have 0 y (x). In particular, as we stated at the beginning of this paper: d g d = y 1 p 1 (d) = p 1 d + d + 1 = d + 1 g d. This completes the proof of the lemma. The final result we shall need is: Lemma : Define the following symmetric 5 5 matrix families for d 1, 0 : where, A x = 0 0 f 1 0 f g 1 f g 1 d 0 0 g f 1 g 1 d 0 0, f 1 = f 1 x, d = 1 x + 1 (d + 1 x), g 1 = g 1 x, d = x(x d). 1

15 where, B x, y = 1 u 0 0 u 0 u d + 1 y v u 0 0 v 1 s 0 d + 1 y v 0 0 u v 1 s 0 0, s = s x, y, d = 1 + x + y + d x y + d 1, u = u x, y, d = (w 1 x)(x w )(x + y), x v = v x, y, d = (x + y)(x + 1)(x d)(x d 1)(x + y + 1)(x + y d 1), w 1 = w 1 x, y, d = 1 d + 1 d + d + 1 x x + y, w = w x, y, d = 1 d 1 d + d + 1 x x + y. Then A x is nonnegative for any x within the line segment OJ,. If d 1, + 5 y > 0 within the triangle OBJ, then B(x, y) is nonnegative for any pair x, y with. If d + 5, 0 then B(x, y) is nonnegative for any pair x, y with y > 0 within the -vertex shape OHIJ, which is formed by the intersection of the shape P with the triangle OBJ,. The spectrum of A x is σ = 1, x, 0, d x, d 1 for any x within the line segment OJ, 5. The spectrum of B x, y is σ = 1, x, y, d x y, d 1 for any pair x, y with y > 0 within the triangle OBJ. Proof: A point x, y within the triangle OBJ satisfies the following conditions: y x, y x + d + 1, y 0.

16 Using these conditions and the fact that d 0 it is trivial to show that all the matrix elements of A(x) are nonnegative. For y > 0 we get from the above conditions 0 < x < d + 1. Also, as y > 0 we have d 1. Since 1 < d 0, we get d d d + 1 = (d + 1) d d + 1 (d + 1) > d + 1 x. Therefore, d + 1 > d + 1. x d Using this result and by the second triangle condition y x + d + 1 < x + d + 1 x d, x + y < d + 1 x d, x x d < x d + 1 x + y, (x d) < d + d + 1 x x + y, 0 < x d < d + d + 1 x x + y, x < 1 d + 1 d + d + 1 x x + y = w 1. Therefore, we showed that x < w 1 for a point x, y with y > 0 within the triangle OBJ. By the triangle conditions, y > 0, and 1 < d 0 it can be easily shown that u > 0 and that all the matrix elements of B(x, y), apart from 1 s, are nonnegative. By Lemma we know that s 0 within the given regions for which B(x, y) is claimed to be nonnegative. The characteristic polynomial of A(x) is: 16

17 p A z = z 5 g 1 + f 1 + d z + df 1 z + g 1 + f 1 g 1 z = z 5 x x d + x + 1 (d + 1 x) + d z + d x + 1 d + 1 x z + x x d + x x d x + 1 d + 1 x z = z 5 x dx + d + d + 1 z + dx + d x + d + d z + x + dx d x dx z = z 1 z x z z d x z d 1. This shows that the spectrum of A(x) is σ = 1, x, 0, d x, d 1. We shall now compare the coefficients of the characteristic polynomial of B(x, y) and the coefficients of the polynomial q z = z 1 z x z y z d x y z d 1 As before let = z 5 + q z + q z + q z + q 1 z + q 0. s k = 1 + x k + y k + d x y k + d 1 k. Obviously, s 1 = 0 and q = 0. By the Newton identities: q = q s 1 s = s, q = q s 1 q s + s = s, q 1 = q s 1 q s q s s = q s s, 5q 0 = q 1 s 1 + q s + q s + q s + s 5 = q s + q s + s 5. We therefore get: q = 1 s, q = 1 s, q 1 = 1 s s, q 0 = 1 6 s s 1 5 s 5. The characteristic polynomial of B(x, y) is: p B z = z 5 + p z + p z + p 1 z + p 0, 17

18 where, p = (d + 1)y u v u u s 6u = 1 u d + 1 y + v + 8u s, p = u s 6u = 1 s = q, (d + 1)y p 1 = v s u u 6u + v u + + (d + 1)y u (d + 1)y u s 6u + v u u u = 1 16u 8 d + 1 yv s + v + 8 d + 1 y u d + 1 y s + 8v u 6, (d + 1)y p 0 = v u u u + (d + 1)y u u s 6u = 1 u d + 1 yv + 1 d + 1 y s. Substituting w 1 and w in the function u gives: u = 1 d x x + y + d + 1 = 1 x xy + dx + dy + d + 1. The following expressions are polynomials in x and y since u and v appear only with even powers. Therefore, it is straightforward to check that u p = d + 1 y + v + 8u s = u s = u q, 16u 8 p 1 = d + 1 yv s + v + 8 d + 1 y u d + 1 y s + 8v u 6 = u 8 s + s = 16u 8 q 1, u p 0 = d + 1 yv + 1 d + 1 y s = u 1 6 s s 1 5 s 5 = u q 0. Since u > 0 we get that all the coefficient of q z and p B z are equal, so the spectrum of B x, y is σ = 1, x, y, d x y, d 1. This completes the proof of the lemma. 18

19 5. Proof of main result Proof of Theorem (Main result): We shall prove the theorem by considering three different cases: 1. x 0,. x > 0 and y 0,. x > 0 and y > 0. Note that the triangle ABC for d, 1 is fully covered by case 1 and case. For d 1, + 5 case is the triangle OBJ without the edge OJ. For d + 5, 0 case is the -vertex shape OHIJ, which is formed by the intersection of the shape P with the triangle OBJ, without the edge OJ. Note that when d = 0 this intersection is empty so case is never happens. Let d, 1. We already know that if σ = 1, x, y, d x y, d 1 is a normalized spectrum of a matrix A R then (x, y) must lie within the triangle ABC. This proves the necessity of the condition. To prove sufficiency, assume that (x, y) lies within the triangle ABC. As mentioned before, case is impossible, so we are left with the other two cases. The triangle ABC conditions are: y x, y x + d + 1, y 1 d x. If x 0 then the conditions of Theorem are satisfied and therefore σ = 1, x, y, d x y, d 1 is realized by a symmetric nonnegative 5 5 matrix A. Since the sum of the elements of σ are zero, then A R. Note that this proof is valid for d, 0. If x > 0 and y 0 then let σ = 1, x, y, d x y, d 1 = λ 1, λ, λ, λ, λ 5 and K 1 =,5, K = {}. We have 5 i=1 λ i = 0, i K 1 λ i = y d 1 and i K λ i = d x y. 19

20 Assume that i K1 λ i < i K λ i. In that case d + 1 y < x + y d. Therefore, y > x + d + 1. As y 0 we have y y. Using the second triangle condition we get x + d + 1 y y > x + d + 1, which is a contradiction and therefore, i K1 λ i i K λ i. As we assumed that d 1 we have d + d. From the second and third triangle conditions we have x d + and y 1 (d x). Therefore, y 1 d x 1 d 1 d + 1 d + 1 d = d. This means that 1 d + 1 y = i K1 λ i. All the conditions of Theorem 5 are satisfied and therefore we conclude that σ = 1, x, y, d x y, d 1 is realized by symmetric nonnegative 5 5 matrix A. Since the sum of the elements of σ are zero, then A R. Let d 1, + 5. We already know that if σ = 1, x, y, d x y, d 1 is a normalized spectrum of a matrix A R then (x, y) must lie within the quadrangle ABFG. This proves the necessity of the condition. To prove sufficiency, assume that (x, y) lies within the quadrangle ABFG. We deal with case 1 and case and leave case for later. The quadrangle ABFG conditions are: y x, y x + d + 1, y 1 d x, x d + 1. Obviously these are triangle ABC conditions plus the MN condition. Case 1 is already proved as noted above. Case is proved using Theorem 5 as before. We assume x > 0 and y 0. If x > 0 and y 0 then let σ = 1, x, y, d x y, d 1 = λ 1, λ, λ, λ, λ 5 and 5 K 1 =,, K = {5}. We have i=1 λ i = 0, i K 1 λ i = d x, i K λ i = d 1. First assume that x > d + 1. As d 1 we get x > 0. By the fourth quadrangle inequality x d + 1, so 1 x d = i K1 λ i. 0

21 By x > d + 1 we get i K 1 λ i = x d > d + 1 = i K λ i. All the conditions of Theorem 5 are satisfied and therefore we conclude that σ = 1, x, y, d x y, d 1 is realized by symmetric nonnegative 5 5 matrix A. Since the sum of the elements of σ are zero, then A R. Next assume that 0 < x d + 1. As d 0 we have 1 d + 1 = i K λ i. By x d + 1 we get i K 1 λ i = x d d + 1 = i K λ i. Again, all the conditions of Theorem 5 are satisfied (with the roles of K 1, K switched) and therefore σ = 1, x, y, d x y, d 1 is realized by symmetric nonnegative 5 5 matrix A. Since the sum of the elements of σ are zero, then A R. Note that this proof is valid for d 1, 0. Let d + 5, 0. By Lemma we know that if σ = 1, x, y, d x y, d 1 is a normalized spectrum of a matrix A R then (x, y) must lie within the shape P. This proves the necessity of the condition. To prove sufficiency, assume that (x, y) lies within the shape P. Case 1 and case are already proved for this range of d as noted above. By Lemma we know that for any pair (x, y) which meets case for d 1, 0 there is a matrix B(x, y) R with a spectrum σ = 1, x, y, d x y, d 1. Therefore, we proved sufficiency. This completes the proof of the theorem. Acknowledgement I wish to thank Raphi Loewy for his helpful comments after reading an earlier draft of this paper. Appendix A The following figures show the points defined in Section for different values of d. 1

22 Figure 1 d, 1 Figure d = 1 Figure d 1, 1

23 Figure d = 1 Figure 5 d 1, + 5 Figure 6 d = + 5

24 Figure 7 d + 5, 0 Figure 8 d = 0 References [1] A. Berman, R.J. Plemmons, Nonnegative Matrices in the Mathematical Sciences, SIAM, Philadelphia, 199. [] M. Fiedler, Eigenvalues of nonnegative symmetric matrices. Linear Algebra and its Applications, 9:119-1, 197. [] C.R. Johnson, T.J. Laffey, R. Loewy, The real and the symmetric nonnegative inverse eigenvalue problems are different, Proc. AMS 1: , [] T. Laffey, E. Meehan, A characterization of trace zero nonnegative 5x5 matrices. Linear Algebra and its Applications, 0-0:95-0, [5] R. Loewy, Unpublished. [6] R. Loewy, D. London, A note on the inverse eigenvalue problem for nonnegative matrices. Linear and Multilinear Algebra 6:8-90, 1978.

25 [7] R. Loewy, J.J. McDonald, The symmetric nonnegative inverse eigenvalue problem for 5x5 matrices. Linear Algebra and its Applications, 9:75-98, 00. [8] J.J.McDonald, M. Neumann, The Soules approach to the inverse eigenvalues problem for nonnegative symmetric matrices of order n 5. Contemporary Mathematics 59:87-07, 000. [9] R. Reams, An inequality for nonnegative matrices and the inverse eigenvalue problem. Linear and Multilinear Algebra 1:67-75, [10] H. R. Suleimanova, Stochastic matrices with real characteristic values, Dokl. Akad. Nauk SSSR, 66:-5,

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