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 A185978 Nontriangular numbers which are the sum of two (positive) triangular numbers. 3
 2, 4, 7, 9, 11, 12, 13, 16, 18, 20, 22, 24, 25, 27, 29, 30, 31, 34, 37, 38, 39, 42, 43, 46, 48, 49, 51, 56, 57, 58, 60, 61, 64, 65, 67, 69, 70, 72, 73, 76, 79, 81, 83, 84, 87, 88, 90, 92, 93, 94, 97, 99, 100, 101, 102, 106, 108 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS This is A051533 (sums of two positive triangular numbers) excluding triangular numbers. This is also A020756 (sums of two triangular numbers) excluding triangular numbers. There may be multiple representations, e.g., 16 = 1 + 15 = 6 + 10. LINKS G. C. Greubel, Table of n, a(n) for n = 1..5000 FORMULA This is the sorted, made unique set {binomial(k+1,2) + binomial(L+1,2), 1 <= k <= L sufficiently large}, excluding members from A000217 (triangular numbers). EXAMPLE a(6) = 12 = 6 + 6, a(17) = 31 = 3 + 28 = 10 + 21. MATHEMATICA (* First run the program for A051533 *) Complement[%, tri] (* Alonso del Arte, Feb 15 2011 *) CROSSREFS Cf. A000217, A020756 (sums of two triangular numbers), A051533 (sums of two positive triangular numbers). Sequence in context: A047541 A308496 A226812 * A351799 A120749 A187978 Adjacent sequences: A185975 A185976 A185977 * A185979 A185980 A185981 KEYWORD nonn,easy AUTHOR Wolfdieter Lang, Feb 14 2011 STATUS approved

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Last modified December 6 01:58 EST 2023. Contains 367594 sequences. (Running on oeis4.)