A354926 - OEIS

%I #13 Jun 13 2022 16:47:28

%S 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1,0,0,0,0,

%T 0,0,0,0,0,0,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1,0,0,

%U 0,1,0,0,0,0,0,0,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1,0,0,1,0,0,0,0,1,0,0,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1

%N a(n) = 1 if n is a product of three distinct primes, otherwise 0.

%H Antti Karttunen, <a href="/A354926/b354926.txt">Table of n, a(n) for n = 1..100000</a>

%H <a href="/index/Ch#char_fns">Index entries for characteristic functions</a>

%H <a href="/index/Eu#epf">Index entries for sequences computed from exponents in factorization of n</a>

%F a(n) = [A001221(n) == 3] * [A001222(n) == 3], where [ ] is the Iverson bracket.

%F a(n) = A008966(n) * A101605(n).

%F For all n >= 1, a(n) <= A340369(n).

%t a[n_] := If[FactorInteger[n][[;; , 2]] == {1, 1, 1}, 1, 0]; Array[a, 100] (* _Amiram Eldar_, Jun 13 2022 *)

%o (PARI) A354926(n) = ((3==bigomega(n)) && (3==omega(n)));

%Y Characteristic function of A007304.

%Y Cf. A001221, A001222, A008966, A101605, A340369.

%K nonn

%O 1

%A _Antti Karttunen_, Jun 13 2022