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 A005090 Number of primes = 2 mod 3 dividing n. 3
 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, 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, 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,10 LINKS Antti Karttunen, Table of n, a(n) for n = 1..10000 FORMULA Additive with a(p^e) = 1 if p = 2 (mod 3), 0 otherwise. From Antti Karttunen, Jul 10 2017: (Start) a(1) = 0; for n > 1, floor((A020639(n) mod 3)/2) + a(A028234(n)). a(n) = A001221(n) - A005088(n) - A079978(n). (End) MATHEMATICA Array[DivisorSum[#, 1 &, And[PrimeQ@ #, Mod[#, 3] == 2] &] &, 120] (* Michael De Vlieger, Jul 11 2017 *) PROG (Scheme) (define (A005090 n) (if (= 1 n) 0 (+ (A004526 (modulo (A020639 n) 3)) (A005090 (A028234 n))))) ;; Antti Karttunen, Jul 10 2017 (PARI) a(n) = my(f=factor(n)); sum(k=1, #f~, (f[k, 1] % 3) == 2); \\ Michel Marcus, Jul 11 2017 (Python) from sympy import primefactors def a(n): return sum([1 for p in primefactors(n) if p%3==2]) print map(a, range(1, 101)) # Indranil Ghosh, Jul 11 2017 CROSSREFS Cf. A001221, A005074, A005088, A079978. Sequence in context: A191904 A265892 A324966 * A073490 A279907 A225654 Adjacent sequences:  A005087 A005088 A005089 * A005091 A005092 A005093 KEYWORD nonn AUTHOR EXTENSIONS More terms from Antti Karttunen, Jul 10 2017 STATUS approved

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Last modified January 20 05:26 EST 2020. Contains 331067 sequences. (Running on oeis4.)