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 A088289 Triangular numbers in which the sum of the external digits equals the sum of the internal digits. 1
 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 (list; graph; refs; listen; history; text; internal format)
 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

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Last modified December 7 04:56 EST 2023. Contains 367629 sequences. (Running on oeis4.)