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%I #6 Feb 12 2021 06:15:13
%S 1,3,6,10,55,66,210,253,666,780,4851,8911,15400,17766,125751,303810,
%T 407253,906531,1125750,2648451,5925403,6706953,15772536,22207780,
%U 50085036,78406503,438094800,1623331710,1764803755,3437158416
%N Triangular numbers that remain triangular when the least significant digit is moved to the beginning.
%e 8911 = A000217(133) is in the sequence because 8911 becomes 1891 which is A000217(61).
%t Do[k = FromDigits[ RotateRight[ IntegerDigits[n(n + 1)/2]]]; l = Floor[ Sqrt[2k]]; If[k == l(l + 1)/2, Print[n(n + 1)/2]], {n, 1, 10^5}]
%Y Cf. A000217, A067269.
%K base,nonn
%O 1,2
%A _Robert G. Wilson v_, Feb 28 2002