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 A020756 Numbers that are the sum of two triangular numbers. 17
 0, 1, 2, 3, 4, 6, 7, 9, 10, 11, 12, 13, 15, 16, 18, 20, 21, 22, 24, 25, 27, 28, 29, 30, 31, 34, 36, 37, 38, 39, 42, 43, 45, 46, 48, 49, 51, 55, 56, 57, 58, 60, 61, 64, 65, 66, 67, 69, 70, 72, 73, 76, 78, 79, 81, 83, 84, 87, 88, 90, 91, 92, 93, 94, 97, 99, 100, 101, 102, 105, 106, 108 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS The possible sums of a square and a promic, i.e., x^2+n(n+1), e.g., 3^2 + 2*3 = 9 + 6 = 15 is present. - Jon Perry, May 28 2003 A052343(a(n)) > 0; union of A118139 and A119345. - Reinhard Zumkeller, May 15 2006 Also union of A051533 and A000217. - Ant King, Nov 29 2010 LINKS T. D. Noe, Table of n, a(n) for n = 1..10000 John A. Ewell, On Sums of Triangular Numbers and Sums of Squares, The American Mathematical Monthly, 99:8 (October 1992), pp. 752-757. T. Kahovanova, K. Knop, A. Radul, Baron Munchhausen's Sequence, J. Int. Seq. 13 (2010) # 10.8.7 FORMULA Numbers n such that 4n+1 is the sum of two squares, i.e. such that 4n+1 is in A001481. Hence n is a member if and only if 4n+1 = odd square * product of distinct primes of form 4k+1. (Fred Helenius and others, Dec 18 2004) Equivalently, we may say that a positive integer n can be partitioned into a sum of two triangular numbers if and only if every 4 k + 3 prime factor in the canonical form of 4 n + 1 occurs with an even exponent. - Ant King, Nov 29 2010 Also, the values of n for which 8n+2 can be partitioned into a sum of two squares of natural numbers. - Ant King, Nov 29 2010 Closed under the operation f(x, y) = 4*x*y + x + y. MATHEMATICA q[k_] := If[! Head[Reduce[m (m + 1) + n (n + 1) == 2 k && 0 <= m && 0 <= n, {m, n}, Integers]] === Symbol, k, {}]; DeleteCases[Table[q[i], {i, 0, 108}], {}] (* Ant King, Nov 29 2010 *) Take[Union[Total/@Tuples[Accumulate[Range[0, 20]], 2]], 80] (* Harvey P. Dale, May 02 2012 *) PROG (PARI) v=vector(200); vc=0; for (x=0, 10, for (y=0, 10, v[vc++ ]=x^2+y*(y+1))); v=vecsort(v); v (PARI) is(n)=my(f=factor(4*n+1)); for(i=1, #f~, if(f[i, 1]%4==3 && f[i, 2]%2, return(0))); 1 \\ Charles R Greathouse IV, Jul 05 2013 (Haskell) a020756 n = a020756_list !! (n-1) a020756_list = filter ((> 0) . a052343) [0..] -- Reinhard Zumkeller, Jul 25 2014 CROSSREFS Complement of A020757. Cf. A051533 (sums of two positive triangular numbers, A001481 (sums of two squares), A002378, A000217. Cf. A052343. Sequence in context: A039148 A065904 A039108 * A051382 A026514 A285974 Adjacent sequences:  A020753 A020754 A020755 * A020757 A020758 A020759 KEYWORD nonn,nice AUTHOR EXTENSIONS Entry revised by N. J. A. Sloane, Dec 20 2004 STATUS approved

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Last modified February 29 08:26 EST 2020. Contains 332355 sequences. (Running on oeis4.)