A358071 - OEIS

A358071

Numbers k that can be written as the sum of a perfect square and a factorial in at least 2 distinct ways.

2, 6, 10, 124, 145, 220, 649, 745, 1081, 1249, 1345, 2929, 3601, 3745, 5065, 5076, 5161, 5209, 5481, 6049, 6196, 6265, 6804, 7249, 7945, 8289, 9529, 11124, 14644, 15649, 17361, 17809, 21169, 22921, 30649, 35316, 40321, 40384, 40720, 40761, 43456, 43569, 43801

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OFFSET

1,1

COMMENTS

This does not count x^2 and (-x)^2 as distinct, nor does it count 0! and 1! as distinct.

For any two factorials a > b, where a-b = m*n where m > n and (m and n are both even or m and n are both odd), (((m-n)/2)^2 + a) will appear in this sequence.

LINKS

Table of n, a(n) for n=1..43.

EXAMPLE

145 = 5^2 + 5! = 11^2 + 4! = 12^2 + 1!.

MATHEMATICA

With[{f = Range[8]!}, c[n_] := Count[f, _?(IntegerQ @ Sqrt[n - #] &)]; Select[Range[f[[-1]]], c[#] > 1 &]] (* Amiram Eldar, Oct 30 2022 *)

CROSSREFS

Cf. A000142, A000290.

Sequence in context: A083458 A124621 A325237 * A065799 A162582 A123098

Adjacent sequences: A358068 A358069 A358070 * A358072 A358073 A358074

KEYWORD

nonn

AUTHOR

Walter Robinson, Oct 30 2022

STATUS

approved