

A180590


Numbers n such that n! is the sum of two triangular numbers.


2



0, 1, 2, 3, 4, 5, 7, 8, 9, 10, 13, 15, 16, 17, 21, 24, 27, 28, 29, 32, 33, 34, 42, 49, 54, 59, 66, 68, 72, 79, 80, 81, 85, 86, 95, 96, 99, 102
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OFFSET

1,3


COMMENTS

Numbers z such that there are nonnegative numbers x and y such that x(x+1)/2 + y(y+1)/2 = z!. Equivalently, (2x+1)^2+(2y+1)^2 = 8z!+2. A necessary and sufficient condition for this is that all the prime factors of 4z!+1 that are congruent to 3 mod 4 occur to even powers (cf. A001481).
Based on an email from R. K. Guy to the Sequence Fans Mailing List, Sep 10 2010.
See A152089 for further links.


LINKS

Table of n, a(n) for n=1..38.
Factor Database, Factors of the numbers 4z!+1


EXAMPLE

0!=1!=T(0)+T(1), 2!=T(1)+T(1), 3!=T(0)+T(2), 4!=T(2)+T(4), 5!=T(5)=T(14), 7!=T(45)+T(89), 8!=T(89)+T269), 9!=T(210)+T(825), 10!=T(760)+T(2610), 13!= T(71504)+T(85680), 15!=T(213384)+T(1603064), etc.


MATHEMATICA

triQ[n_] := IntegerQ@ Sqrt[8 n + 1]; fQ[n_] := Block[{k = 1, lmt = Floor@Sqrt[2*n! ], nf = n!}, While[k < lmt && ! triQ[nf  k (k + 1)/2], k++ ]; r = (Sqrt[8*(nf  k (k + 1)/2) + 1]  1)/2; Print[{k, r, n}]; If[IntegerQ@r, True, False]]; k = 1; lst = {}; While[k < 69, If[ fQ@ k, AppendTo[lst, k]]; k++ ]; lst


CROSSREFS

A171099 gives the number of solutions. Cf. A000142, A000217, A001481, A076680. Complement of A152089.
Sequence in context: A175020 A050728 A230999 * A129268 A271317 A039169
Adjacent sequences: A180587 A180588 A180589 * A180591 A180592 A180593


KEYWORD

nonn,more


AUTHOR

Robert G. Wilson v, Sep 10 2010


EXTENSIONS

Edited by N. J. A. Sloane, Sep 24 2010
69 eliminated by N. J. A. Sloane, Sep 24 2010 (see A152089).
Extended by G. Guninski and D. S. McNeil, Sep 24 2010
95, 96, 99, 102 from G. Guninski, Oct 12 2010


STATUS

approved



