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 A023520 #16 Jul 07 2022 16:18:37 %S A023520 0,1,1,2,1,2,1,2,1,1,1,2,1,2,1,1,1,1,2,1,1,2,1,3,2,1,2,1,2,1,2,1,1,1, %T A023520 1,1,1,2,1,1,1,1,1,2,1,2,2,2,1,2,1,1,1,1,1,1,1,1,2,1,1,1,2,1,2,1,1,1, %U A023520 1,2,1,3,1,1,2,1,6,2,3 %N A023520 Exponent of 2 in prime factorization of prime(n)*prime(n-1) - 1. %C A023520 a(n) = 1 if and only if A080378(n-1) = 2. - _Robert Israel_, Feb 07 2018 %H A023520 Robert Israel, <a href="/A023520/b023520.txt">Table of n, a(n) for n = 2..10000</a> %F A023520 a(n) = A007814(A023515(n)). - _Michel Marcus_, Sep 30 2013 %p A023520 seq(padic:-ordp(ithprime(n)*ithprime(n-1)-1,2),n=2..200); # _Robert Israel_, Feb 07 2018 %o A023520 (PARI) a(n) = valuation(prime(n)*prime(n-1) - 1, 2); \\ _Michel Marcus_, Sep 30 2013 %o A023520 (Python) %o A023520 from sympy import prime %o A023520 def A023520(n): return (~(m:=prime(n)*prime(n-1)-1)& m-1).bit_length() # _Chai Wah Wu_, Jul 07 2022 %Y A023520 Cf. A007814, A023515, A080378. %K A023520 nonn %O A023520 2,4 %A A023520 _Clark Kimberling_ %E A023520 Offset set to 2 and a(2) corrected by _Michel Marcus_, Sep 30 2013