## S000971

Number of solutions to the Pell equation x^2 - d*y^2 = 1, where x is tribonacci number t(n).

1, 1, 3, 2, 2, 2, 3, 4, 2, 1, 4, 3, 1, 2, 3, 6, 1, 1, 3, 2, 1, 2, 4, 2, 1, 1, 5, 9, 1, 2, 8, 3, 2, 2, 6, 2, 1, 2, 6, 2, 2, 1, 4, 3, 4, 1, 3, 3, 1, 1, 3, 2, 4, 1, 8, 2, 1, 2, 9, 4, 1, 2, 4, 4, 1, 1, 6, 4, 4, 2, 3, 4, 2, 2, 4, 3, 2, 2, 3, 6, 1, 1, 12, 8, 2, 1, 4

1

T. D. Noe, Plot of terms 4..128

T. D. Noe, Table of terms 4..128

Florian Luca, Amanda Montejano, Laszlo Szalay, and Alain Togbe, On the x-coordinates of Pell equations which are tribonacci numbers, arXiv 1612.09546 (Dec 30 2016)

(Mma) nn = 64; s1 = Table[0, {nn}]; s2 = s1; tri = {0, 0, 1, 1}; t = Table[AppendTo[tri, tri[[-1]] + tri[[-2]] + tri[[-3]]]; x = tri[[-1]]; s1[[n]] = FindInstance[x^2 - d*y^2 == 1 && d > 1 && y > 0, {d, y}, Integers, 20]; s2[[n]] = FindInstance[x^2 - d*y^2 == -1 && d > 1 && y > 0, {d, y}, Integers, 20]; s1[[n]] = Table[{s1[[n, i, 1, 2]], s1[[n, i, 2, 2]]}, {i, Length[s1[[n]]]}]; s2[[n]] = Table[{s2[[n, i, 1, 2]], s2[[n, i, 2, 2]]}, {i, Length[s2[[n]]]}]; {Length[s1[[n]]], Length[s2[[n]]]}, {n, 4, nn}]; Table[t[[n, 1]], {n, nn - 3}]

Cf. S000970.

nonn

T. D. Noe, Feb 24 2017