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Smallest number representable as the sum of 3 triangular numbers in exactly n ways.
10

%I #17 Feb 25 2015 04:21:41

%S 0,3,12,21,52,57,91,121,136,211,192,226,409,331,367,406,511,507,886,

%T 637,772,721,871,952,1102,1066,1227,1192,1641,1621,1396,1381,1501,

%U 1732,1792,1927,1942,2401,2611,2551,2422,2557,2887,2821,3136,3271,3607,3376

%N Smallest number representable as the sum of 3 triangular numbers in exactly n ways.

%C Fermat claimed, Euler tried, Gauss proved (July 10, 1796) that every number can be represented as a sum of three triangular numbers. I'm considering 0 as a triangular number here. If at first you do not succeed, tri + tri + tri again.

%C Conjecture: for n large enough, 1 < a(n)/n^2 < 2. - _Benoit Cloitre_, May 10 2003

%C Conjecture: No term a(n) with n > 2 is congruent to 0 or 3 modulo 5. - _Zhi-Wei Sun_, Feb 25 2015

%H T. D. Noe, <a href="/A061262/b061262.txt">Table of n, a(n) for n=1..1000</a>

%e 57 is the smallest number that can be represented by exactly 6 different triangular triple sums: {6, 6, 5}, {7, 7, 1}, {8, 5, 3}, {8, 6, 0}, {9, 3, 3}, {10, 1, 1}.

%t a = Table[ n(n + 1)/2, {n, 0, 85} ]; b = {0}; c = Table[0, {3655} ]; Do[ b = Append[b, a[[i] ] + a[[j]] + a[[k]]], {k, 1, 85}, {j, 1, k}, {i, 1, j} ]; b = Delete[b, 1]; b = Sort[b]; l = Length[b]; Do[ If[b[[n]] < 3655, c[[b[[n]] + 1]]++ ], {n, 1, l} ]; Do[ k = 1; While[ c[[k]] != n, k++ ]; Print[k - 1], {n, 1, 48} ]

%Y Cf. A000217, A053614, A060773, A002636.

%Y Cf. A124978.

%K easy,nice,nonn

%O 1,2

%A _Ed Pegg Jr_, Apr 24 2001