OFFSET
1,1
COMMENTS
Values of k in A215658.
See A161620 for the primorial values. - Jeppe Stig Nielsen, Mar 27 2018
LINKS
Carol Nelson, David E. Penney and Carl Pomerance, 714 and 715, J. Recreational Math. 7:2 (1994), pp. 87-89.
FORMULA
a(n) * (a(n) - 1) = A215658(n)#, where p# = 2 * 3 * 5 * 7 * 11 * ... * p is a primorial, the product of the primes from 2 to p.
MATHEMATICA
Select[Range[10^5], Product[Prime@ i, {i, PrimeNu@ #}] == # &[# (# - 1)] &] (* Michael De Vlieger, Apr 10 2018 *)
PROG
(Python)
from sympy import primorial, integer_nthroot
A215659_list = []
for i in range(1, 10**2):
a, b = integer_nthroot(4*primorial(i)+1, 2)
if b:
A215659_list.append((a+1)//2) # Chai Wah Wu, Apr 01 2021
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Jonathan Sondow, Sep 07 2012
EXTENSIONS
Name improved by Jeppe Stig Nielsen, Mar 27 2018
STATUS
approved