A230363 Factorials representable as b^2 + triangular(c).

1, 1, 2, 6, 24, 120, 362880, 3628800, 39916800, 479001600, 6227020800, 1307674368000, 121645100408832000

Offset: 1

Author

Alex Ratushnyak, Oct 17 2013

Keywords

• nonn
• more

Comments

Numbers n such that n! is representable as a sum of a square and a triangular number: 0, 1, 2, 3, 4, 5, 9, 10, 11, 12, 13, 15, 19, ... .

1! = 1/2*1*(1+1), 3! = 1/2*3*(3+1) and 5! = 1/2*15*(15+1)/2 are triangular terms of the sequence. Next such term, if it exists is greater than 10000!. - Farideh Firoozbakht, Oct 18 2013

Examples

			13! = 66708^2+1/2*59616(59616+1) = 78693^2+1/2*8298(8298+1), so 13! = 6227020800 is in the sequence. What is the next term of the sequence which has more than one representation of the form b^2 + triangular(c)? - _Farideh Firoozbakht_, Oct 18 2013
		

Crossrefs

Cf. A000142, A000290, A000217, A014133.

Programs

Python

import math
f=1
for n in range(1, 1000000):
    f *= n
    t = b = 0
    while t<=f:
        x = f-t
        a = int(math.sqrt(x))
        if a*a==x:
            print(f, end=", ")
            break
        b += 1
        t = b*(b+1)//2

Formula

A014133 INTERSECT A000142. - R. J. Mathar, Oct 11 2014

Extensions

Initial 1 added by Farideh Firoozbakht, Oct 18 2013