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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 (list; graph; refs; listen; history; text; internal format)
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 * A289342 A286302 A129268

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

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Last modified August 17 01:56 EDT 2017. Contains 290631 sequences.