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 A005091 Number of distinct primes = 3 mod 4 dividing n. 10
 0, 0, 1, 0, 0, 1, 1, 0, 1, 0, 1, 1, 0, 1, 1, 0, 0, 1, 1, 0, 2, 1, 1, 1, 0, 0, 1, 1, 0, 1, 1, 0, 2, 0, 1, 1, 0, 1, 1, 0, 0, 2, 1, 1, 1, 1, 1, 1, 1, 0, 1, 0, 0, 1, 1, 1, 2, 0, 1, 1, 0, 1, 2, 0, 0, 2, 1, 0, 2, 1, 1, 1, 0, 0, 1, 1, 2, 1, 1, 0, 1, 0, 1, 2, 0, 1, 1, 1, 0, 1, 1, 1, 2, 1, 1, 1, 0, 1, 2, 0, 0, 1 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,21 COMMENTS a(n) = Sum_{k=1..A001221(n)} A079261(A027748(n,k)); a(A072437(n)) = 0; a(A187811(n)) > 0. - Reinhard Zumkeller, Jan 07 2013 LINKS Reinhard Zumkeller, Table of n, a(n) for n = 1..10000 FORMULA Additive with a(p^e) = 1 if p = 3 (mod 4), 0 otherwise. a(n) = Sum_{p|n} (binomial(p,3) mod 2), where p is a prime. - Ridouane Oudra, Nov 19 2019 MAPLE with(numtheory): seq(add(binomial(p, 3) mod 2, p in factorset(n)), n=1..100); # Ridouane Oudra, Nov 19 2019 MATHEMATICA f[n_]:=Length@Select[If[n==1, {}, FactorInteger[n]], Mod[#[[1]], 4]==3&]; Table[f[n], {n, 102}] (* Ray Chandler, Dec 18 2011 *) PROG (PARI) for(n=1, 100, print1(sumdiv(n, d, isprime(d)*if((d-3)%4, 0, 1)), ", ")) (Haskell) a005091 = sum . map a079261 . a027748_row -- Reinhard Zumkeller, Jan 07 2013 (MAGMA) [0] cat [#[p:p in PrimeDivisors(n)| p mod 4 eq 3]: n in [2..100]]; // Marius A. Burtea, Nov 19 2019 (MAGMA) [0] cat [&+[Binomial(p, 3) mod 2:p in PrimeDivisors(n)]:n in [2..100]]; // Marius A. Burtea, Nov 19 2019 CROSSREFS Cf. A001221, A005089, A005094. Sequence in context: A277778 A255319 A214088 * A276516 A253638 A086831 Adjacent sequences:  A005088 A005089 A005090 * A005092 A005093 A005094 KEYWORD nonn AUTHOR STATUS approved

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Last modified December 7 17:41 EST 2019. Contains 329847 sequences. (Running on oeis4.)