Wikipedia:Reference desk/Archives/Mathematics/2023 July 17

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July 17

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Zeros in the tribonacci sequence

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The tribonacci sequence is defined by a(0) = a(1) = 0, a(2) = 1, a(n) = a(n-1) + a(n-2) + a(n-3). This defines a(n) not only for for n >= 0, but for n < 0 if you rearrange the recursion formula; see OEISA000073 and OEISA057597. We have a(0) = a(1) = 0, but also a(-3) = 0, and somewhat surprisingly a(-16) = 0. Are there any other values of n with a(n) = 0? Of course n would have to be negative. I strongly doubt there any other values, but I strongly suspect it would be very difficult to prove this. Not coincidentally, the tribonacci constant, τ=1.839286755214161132551852..., has an unexpectedly good rational approximation 103/56, with 56τ - 103 = -τ -16. This is reflected in the large entry in its continued fraction expansion [1, 1, 5, 4, 2, 305, ... ] (OEISA019712). I checked values up to a(-100000) but no other 0's appeared. --RDBury (talk) 21:50, 17 July 2023 (UTC)[reply]

  is the sole real root of the polynomial   the other two being formed by a complex conjugate pair with the approximate values of   Denoting these by   and   we have the identity   for some   in which the   are also a conjugate pair. The values for   and   can be solved using three values for   and the corresponding   Note that   while   so when going left on the integer line, the powers of the   are going to dominate. I expect the explicit formula for   can be used to set a bound on   below which no more zeros can appear.  --Lambiam 07:34, 18 July 2023 (UTC)[reply]
I see what you're saying, but ignoring the τ -n term gives an expression of the form ατn/2cos(nβ), where α and β are determined by qi and ξi. (It's easy to see that |ξi| = |τ|-1/2, since τξ1ξ2=1. ln(ξ1/|ξ1|)=-ln(ξ2/|ξ2|)=iβ.) You can bound the ατn/2 below, but the cos(nβ) part can and will get as close to 0 as you want. You could also argue informally that if these bounds were going to work at all then they would work for -16. --RDBury (talk) 14:18, 18 July 2023 (UTC)[reply]
Correction: I should have included a sine term as well as a cosine term. The full expression is:
 
where τ≈1.83928675521416, ζ≈0.182803532968296, ξ=τ−1/2≈0.737352705760328, α1≈-0.182803532968296, α2≈0.681093061654159, β≈2.17623354549187. In any case, it still holds that the trigonometric part can be arbitrarily close to 0 based on β/π being irrational. (There are proofs and references for this in this Stack Exchange post. --RDBury (talk) 01:41, 19 July 2023 (UTC)[reply]

@Lambian: I hope you're not writing things like   instead of   in Wikipedia articles. Michael Hardy (talk) 21:08, 21 July 2023 (UTC)[reply]