This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A215659 #40 Apr 01 2021 17:24:53
%S A215659 2,3,6,15,715
%N A215659 Values of k such that k*(k - 1) is a primorial number.
%C A215659 Values of k in A215658.
%C A215659 See A161620 for the primorial values. - _Jeppe Stig Nielsen_, Mar 27 2018
%H A215659 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 A215659 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 A215659 Select[Range[10^5], Product[Prime@ i, {i, PrimeNu@ #}] == # &[# (# - 1)] &] (* _Michael De Vlieger_, Apr 10 2018 *)
%o A215659 (Python)
%o A215659 from sympy import primorial, integer_nthroot
%o A215659 A215659_list = []
%o A215659 for i in range(1,10**2):
%o A215659 a, b = integer_nthroot(4*primorial(i)+1,2)
%o A215659 if b:
%o A215659 A215659_list.append((a+1)//2) # _Chai Wah Wu_, Apr 01 2021
%Y A215659 Cf. A215658, A161620.
%K A215659 nonn,more
%O A215659 1,1
%A A215659 _Jonathan Sondow_, Sep 07 2012
%E A215659 Name improved by _Jeppe Stig Nielsen_, Mar 27 2018