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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
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
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
KEYWORD
nonn,easy
AUTHOR
Wolfdieter Lang, Feb 14 2011
STATUS
approved