Palindromic triangular numbers in base 7.
1, 3, 6, 141, 333, 606, 11211, 33033, 36363, 60606, 1012101, 1525251, 3251523, 3306033, 3352533, 3546453, 6060606, 100646001, 102252201, 112050211, 142323241, 152020251, 312444213, 321414123, 363030363, 606060606, 15240604251, 32104140123, 33011011033
1
See S000349 for the n that produce these triangular numbers.
T. D. Noe, Plot of 73 terms
T. D. Noe, Table of 73 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]; t7 = ff[7, 30]; Table[FromDigits[IntegerDigits[n*(n + 1)/2, 7]], {n, t7}]
Cf. S000349.
nonn,base,hard
T. D. Noe, Nov 19 2014