Palindromic triangular numbers in base 5.
1, 3, 11, 121, 303, 3003, 30303, 1130311, 1222221, 3030303, 14211241, 30133103, 102121201, 103343301, 303030303, 1022442201, 3014334103, 12221012221, 30303030303, 123000000321, 123011110321, 301131131103, 1120424240211, 1204444444021, 3030303030303
1
See S000347 for the n that produce these triangular numbers.
T. D. Noe, Plot of 52 terms
T. D. Noe, Table of 52 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]; t5 = ff[5, 20]; Table[FromDigits[IntegerDigits[n*(n + 1)/2, 5]], {n, t5}]
Cf. S000347.
nonn,base,hard
T. D. Noe, Nov 19 2014