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 A064131 #19 Jan 18 2019 13:56:37 %S A064131 2,3,2,2,2,2,2,2,4,4,2,4,2,2,4,4,2,8,2,4,2,4,4,2,8,4,4,4,4,8,2,4,2,4, %T A064131 4,2,2,8,4,16,4,4,4,2,8,4,4,8,4,4,8,8,4,4,2,4,8,8,4,4,8,4,8,16,2,2,2, %U A064131 4,8,32,8,32,4,4,8,16,16,2,4,8,8,32,8,16,32,8,32,32,8,8,4 %N A064131 Number of divisors of 3^n + 1 that are relatively prime to 3^m + 1 for all 0 < m < n. %H A064131 Sam Wagstaff, Cunningham Project, <a href="https://homes.cerias.purdue.edu/~ssw/cun/">Factorizations of 3^n-1, n odd, n<540</a> %t A064131 a[0]=2; a[n_] := Length@ Select[Divisors[3^n+1], GCD[Times @@ (3^Range[1, n-1] + 1), #] == 1 &]; Array[a, 91, 0] (* _Giovanni Resta_, Jul 02 2018 *) %o A064131 (PARI) a(n) = if (n==0, 2, sumdiv(3^n+1, d, vecsum(vector(n-1, k, gcd(d, 3^k+1) == 1)) == n-1)); \\ _Michel Marcus_, Jun 24 2018 %Y A064131 Cf. A064132, A064133, A064134, A064135, A064136, A064137. %K A064131 nonn %O A064131 0,1 %A A064131 _Robert G. Wilson v_, Sep 10 2001 %E A064131 a(1) corrected and extended by _Michel Marcus_, Jul 02 2018