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%I #40 Apr 01 2021 17:24:53
%S 2,3,6,15,715
%N Values of k such that k*(k - 1) is a primorial number.
%C Values of k in A215658.
%C See A161620 for the primorial values. - _Jeppe Stig Nielsen_, Mar 27 2018
%H Carol Nelson, David E. Penney and Carl Pomerance, <a href="/A215659/a215659.jpg">714 and 715</a>, J. Recreational Math. 7:2 (1994), pp. 87-89.
%F 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.
%t Select[Range[10^5], Product[Prime@ i, {i, PrimeNu@ #}] == # &[# (# - 1)] &] (* _Michael De Vlieger_, Apr 10 2018 *)
%o (Python)
%o from sympy import primorial, integer_nthroot
%o A215659_list = []
%o for i in range(1,10**2):
%o a, b = integer_nthroot(4*primorial(i)+1,2)
%o if b:
%o A215659_list.append((a+1)//2) # _Chai Wah Wu_, Apr 01 2021
%Y Cf. A215658, A161620.
%K nonn,more
%O 1,1
%A _Jonathan Sondow_, Sep 07 2012
%E Name improved by _Jeppe Stig Nielsen_, Mar 27 2018
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