A068859 a(1) = 3 = 1*3; a(n) = smallest multiple of a(n-1) which is of the form k(k+2).

3, 15, 120, 360, 5040, 201600, 256032000, 4064251968000, 1024126980000259968000, 29365589556381243259975904529351552000, 82421263677202543043706796882184010324019881962069201381742720000

Offset: 1

Author

Amarnath Murthy, Mar 12 2002

Keywords

• nonn

Examples

			120 = 10*12 belongs to this sequence and 360 = 18*20 is the smallest such multiple of 120.
		

Links

• Chai Wah Wu, Table of n, a(n) for n = 1..13

Crossrefs

Cf. A068857, A068858.

Programs

Python

from itertools import islice
from sympy import sqrt_mod_iter
def A068859_gen(): # generator of terms
    a = 3
    while True:
        yield a
        b = a+1
        for d in sqrt_mod_iter(1,a):
            if d==1 or d**2-1 == a:
                d += a
            if d < b:
                b = d
        a = b**2-1
A068859_list = list(islice(A068859_gen(),11)) # Chai Wah Wu, May 05 2024

Extensions

More terms from Sascha Kurz, Mar 23 2002

Edited by Don Reble, Sep 12 2003