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%I #15 Jan 08 2015 05:10:39
%S 15,105,378,780,2145,1485,5460,7875,29403,21945,70125,105570,61425,
%T 37950,255255,306153,61425,667590,749700,522753,1016025,353220,176715,
%U 1471470,1445850,1747515,246753,794430,749700,514605,3499335,2953665,5073705,635628,8382465
%N Smaller member of a pair of triangular numbers whose sum and difference are triangular.
%D Albert H. Beiler, Recreations in the theory of numbers, New York, Dover, (2nd ed.) 1966, p. 197, nr. 8.
%e a(2) = 105, since the pair of triangular numbers 171 = 18*(18+1)/2 and 105 = 14*(14+1)/2 produce the sum 276 = 23*(23+1)/2 and the difference 66 = 11*(11+1)/2 which are both triangular numbers.
%o (PARI) lista(n) = {v = vector(nn, n, n*(n+1)/2); for (n=2, nn, for (k=1, n-1, if (ispolygonal(v[n]+v[k], 3) && ispolygonal(v[n]-v[k], 3), print1(v[k], ", "));););} \\ _Michel Marcus_, Jan 08 2015
%Y Cf. A000217, A185128, A185223, A185233, A185243, A185253, A185257, A185258.
%K nonn
%O 1,1
%A _Martin Renner_, Jan 20 2012
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