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 A303975 #43 Jan 01 2019 07:10:44
%S A303975 0,0,1,0,1,1,1,0,1,1,1,1,2,1,2,0,1,1,1,1,1,1,1,1,1,2,1,1,2,2,1,0,2,1,
%T A303975 2,1,2,1,2,1,1,1,2,1,2,1,2,1,1,1,2,2,1,1,2,1,1,2,1,2,2,1,1,0,2,2,1,1,
%U A303975 2,2,2,1,2,2,2,1,2,2,2,1,1,1,1,1,2,2,2
%N A303975 Number of distinct prime factors in the product of prime indices of n.
%C A303975 A prime index of n is a number m such that prime(m) divides n. The multiset of prime indices of n is row n of A112798.
%C A303975 First appearance of n is A062447(n - 1).
%F A303975 a(n) = A001221(A156061(n)). - _Michel Marcus_, Jan 01 2019
%e A303975 9193 has prime indices {10, 66} with product 660, which has 4 distinct prime factors {2, 3, 5, 11}, so a(9193) = 4.
%t A303975 Table[PrimeNu[If[n==1,1,Times@@PrimePi/@First/@FactorInteger[n]]],{n,100}]
%o A303975 (PARI) a(n) = my(v = factor(n)[, 1]); omega(prod(i = 1, #v, primepi(v[i]))) \\ _David A. Corneth_, Dec 29 2018
%Y A303975 Cf. A000961, A001055, A001221, A002110, A003963, A056239, A112798, A156061, A302242, A302505, A320325.
%K A303975 nonn
%O A303975 1,13
%A A303975 _Gus Wiseman_, Dec 28 2018