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 A128428 #20 Dec 06 2020 10:53:28 %S A128428 1,1,2,1,2,1,2,2,2,1,2,2,3,1,2,1,3,2,2,1,3,2,3,1,2,1,3,2,2,2,3,2,3,2, %T A128428 2,1,3,2,2,1,2,2,3,2,2,2,4,2,2,2,2,2,3,1,3,1,3,2,2,2,2,2,3,2,2,1,3,2, %U A128428 2,2,2,3,4,1,3,2,3,2,2,2,3,2,4,1,2,2,3,2,3,1,3,2,3,1,2,2,3,3,3,2,2,2 %N A128428 Number of distinct prime factors of n^2+1. %C A128428 a(n) is also the number of distinct prime factors, up to multiplication by units, of n + i in the ring of Gaussian integers. - _Jason Kimberley_, Dec 17 2011 %H A128428 Charles R Greathouse IV, <a href="/A128428/b128428.txt">Table of n, a(n) for n = 1..10000</a> %F A128428 a(n) = A001221(n^2+1). %e A128428 a(3) = 2 because 3^2+1 = 2*5. %p A128428 a:= n-> nops(select(isprime, numtheory[divisors](n^2+1))): %p A128428 seq(a(n), n=1..100); # _Alois P. Heinz_, Dec 06 2020 %t A128428 a[n_]:=Length[FactorInteger[n^2 + 1]] %o A128428 (PARI) a(n)=omega(n^2+1) \\ _Charles R Greathouse IV_, Jul 31 2011 %Y A128428 Cf. A193330 (counted with multiplicity). %K A128428 nonn %O A128428 1,3 %A A128428 Kent Horvath (kenthorvath(AT)gmail.com), May 10 2007