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 A005090 #28 Apr 14 2021 22:23:48 %S A005090 0,1,0,1,1,1,0,1,0,2,1,1,0,1,1,1,1,1,0,2,0,2,1,1,1,1,0,1,1,2,0,1,1,2, %T A005090 1,1,0,1,0,2,1,1,0,2,1,2,1,1,0,2,1,1,1,1,2,1,0,2,1,2,0,1,0,1,1,2,0,2, %U A005090 1,2,1,1,0,1,1,1,1,1,0,2,0,2,1,1,2,1,1,2,1,2,0,2,0,2,1,1,0,1,1,2,1,2,0,1,1,2,1,1,0,3,0,1,1,1,2,2,0,2,1,2 %N A005090 Number of primes == 2 mod 3 dividing n. %H A005090 Antti Karttunen, <a href="/A005090/b005090.txt">Table of n, a(n) for n = 1..10000</a> %F A005090 Additive with a(p^e) = 1 if p = 2 (mod 3), 0 otherwise. %F A005090 From _Antti Karttunen_, Jul 10 2017: (Start) %F A005090 a(1) = 0; for n > 1, floor((A020639(n) mod 3)/2) + a(A028234(n)). %F A005090 a(n) = A001221(n) - A005088(n) - A079978(n). %F A005090 (End) %t A005090 Array[DivisorSum[#, 1 &, And[PrimeQ@ #, Mod[#, 3] == 2] &] &, 120] (* _Michael De Vlieger_, Jul 11 2017 *) %o A005090 (Scheme) (define (A005090 n) (if (= 1 n) 0 (+ (A004526 (modulo (A020639 n) 3)) (A005090 (A028234 n))))) ;; _Antti Karttunen_, Jul 10 2017 %o A005090 (PARI) a(n) = my(f=factor(n)); sum(k=1, #f~, (f[k,1] % 3) == 2); \\ _Michel Marcus_, Jul 11 2017 %o A005090 (Python) %o A005090 from sympy import primefactors %o A005090 def a(n): return sum(1 for p in primefactors(n) if p%3==2) %o A005090 print([a(n) for n in range(1, 101)]) # _Indranil Ghosh_, Jul 11 2017 %Y A005090 Cf. A001221, A005074, A005088, A079978. %K A005090 nonn %O A005090 1,10 %A A005090 _N. J. A. Sloane_ %E A005090 More terms from _Antti Karttunen_, Jul 10 2017