A088289 Triangular numbers in which the sum of the external digits equals the sum of the internal digits.

231, 253, 561, 990, 2211, 4095, 5050, 5151, 6903, 9180, 11325, 11628, 20301, 20503, 31626, 41041, 51040, 53301, 63546, 66066, 67528, 76245, 92665, 95703, 97020, 98346, 99235, 130305, 131328, 161028, 203203, 313236, 343206, 416328, 500500

Offset

0,1

Links

Harvey P. Dale, Table of n, a(n) for n = 0..202 (all terms up to and including the one millionth triangular number)

Example

4095 is a term: 4 + 5 = 9 = 0 + 9.

Maple

a:=proc(n) local nn: nn:=convert(n*(n+1)/2, base, 10): if sum(nn[j], j=2..nops(nn)-1)=nn[1]+nn[nops(nn)] then n*(n+1)/2 else fi end: seq(a(n), n=1..1300); # Emeric Deutsch, Aug 06 2005

Mathematica

Do[d = IntegerDigits[n*(n+1)/2]; l = Length[d]; s = d[[1]] + d[[l]]; If[Plus @@ d == 2*s, Print[n*(n+1)/2]], {n, 1, 10^3}] (* Ryan Propper, Aug 06 2005 *)
edidQ[n_]:=Module[{idn=IntegerDigits[n]}, Total[idn[[1]]+idn[[-1]]]== Total[ Most[ Rest[idn]]]]; Select[Accumulate[Range[2000]], edidQ]//Quiet (* Harvey P. Dale, Nov 23 2022 *)

Crossrefs

Sequence in context: A031965 A316095 A345795 * A046009 A350367 A337231

Adjacent sequences: A088286 A088287 A088288 * A088290 A088291 A088292

Keyword

base,nonn

Author

Amarnath Murthy, Sep 29 2003

Extensions

Corrected and extended by David Wasserman, Emeric Deutsch and Ryan Propper, Aug 06 2005

Status

approved