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Triangular numbers which remain triangular when the most significant digit is moved to the end.
1

%I #12 Sep 17 2024 12:34:35

%S 1,3,6,10,55,66,300,325,666,1035,1485,1891,61776,103740,109746,112575,

%T 190653,340725,1026028,1264845,3000025,3002475,3592540,3670695,

%U 37840650,60000535,61577253,65008503,1001393128,1432436050,1560756385

%N Triangular numbers which remain triangular when the most significant digit is moved to the end.

%e 1485 is a term as 1485 and 4851 both are triangular numbers.

%t Do[k = FromDigits[ RotateLeft[ IntegerDigits[n(n + 1)/2]]]; l = Floor[ Sqrt[2k]]; If[k == l(l + 1)/2, Print[n(n + 1)/2]], {n, 1, 10^5}]

%t msdmeQ[n_]:=IntegerQ[(Sqrt[1+8*FromDigits[RotateLeft[IntegerDigits[ n]]]]-1)/2]; Select[Accumulate[Range[55900]],msdmeQ] (* _Harvey P. Dale_, Aug 22 2014 *)

%Y Cf. A000217, A345110.

%K base,nonn

%O 1,2

%A _Amarnath Murthy_, Feb 22 2002

%E Edited by _Robert G. Wilson v_, Feb 28 2002

%E Offset changed by _Andrew Howroyd_, Sep 17 2024