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 A281856 #17 Feb 25 2025 14:56:13 %S A281856 1,1,2,2,2,3,2,3,2,4,5,6,3,6,4,3,5,6,4,8,6,10,6,9,4,9,8,12,5,8,11,6,6, %T A281856 10,9,15,6,8,6,16,14,10,6,18,11,15,18,8,15,10,8,12,12,9,10,18,12,9,22, %U A281856 14,18,24,8,20,15,9,16,21,12,10,27,18,16,11,12,23 %N A281856 One fourth of the order of the abelian non-cyclic groups (Z/A033949(n)*Z)^x. %C A281856 a(n) is one fourth of the row product of the irregular triangle A281854. %F A281856 a(n) = A000010(A033949(n))/4, n >= 1. %t A281856 EulerPhi@ Select[Range[2, 130], ! IntegerQ@ PrimitiveRoot@ # &]/4 (* _Michael De Vlieger_, Feb 02 2017 *) %o A281856 (Python) %o A281856 from sympy import primepi, integer_nthroot, totient %o A281856 def A281856(n): %o A281856 def f(x): return int(n+1+(x>=2)+(x>=4)+sum(primepi(integer_nthroot(x,k)[0])-1 for k in range(1,x.bit_length()))+sum(primepi(integer_nthroot(x>>1,k)[0])-1 for k in range(1,x.bit_length()-1))) %o A281856 m, k = n, f(n) %o A281856 while m != k: m, k = k, f(k) %o A281856 return totient(m)>>2 # _Chai Wah Wu_, Feb 25 2025 %Y A281856 Cf. A000010, A033949, A060679, A281854. %K A281856 nonn %O A281856 1,3 %A A281856 _Wolfdieter Lang_, Feb 02 2017