Palindromic triangular numbers in base 9.
1, 3, 6, 11, 111, 161, 363, 646, 1111, 6226, 11111, 16661, 31613, 64246, 111111, 1032301, 1075701, 1111111, 3116113, 3678763, 6382836, 6428246, 10477401, 11111111, 111111111, 167888761, 306272603, 311161113, 316070613, 362525263, 666707666, 1107557011
1
See S000351 for the n that produce these triangular numbers.
T. D. Noe, Plot of 90 terms
T. D. Noe, Table of 90 terms
Eric W. Weisstein, MathWorld: Triangular Number
(Mma) ff[base_, len_] := Module[{n = 0, t = {}, tri, d}, While[Length[t] < len, n++; tri = n*(n + 1)/2; d = IntegerDigits[tri, base]; If[d == Reverse[d], AppendTo[t, n]]]; t]; t9 = ff[9, 40]; Table[FromDigits[IntegerDigits[n*(n + 1)/2, 9]], {n, t9}]
Cf. S000351.
nonn,base,hard
T. D. Noe, Nov 19 2014