cp's OEIS Frontend

A318990 Numbers of the form prime(x) * prime(y) where x divides y.

%I A318990 #11 Nov 04 2022 19:24:39
%S A318990 4,6,9,10,14,21,22,25,26,34,38,39,46,49,57,58,62,65,74,82,86,87,94,
%T A318990 106,111,115,118,121,122,129,133,134,142,146,158,159,166,169,178,183,
%U A318990 185,194,202,206,213,214,218,226,235,237,254,259,262,267,274,278,289
%N A318990 Numbers of the form prime(x) * prime(y) where x divides y.
%H A318990 Andrew Howroyd, <a href="/A318990/b318990.txt">Table of n, a(n) for n = 1..10000</a>
%e A318990 The sequence of all dividing pairs (columns) begins:
%e A318990   1  1  2  1  1  2  1  3  1  1  1  2  1  4  2  1  1  3  1  1  1  2  1  1
%e A318990   1  2  2  3  4  4  5  3  6  7  8  6  9  4  8 10 11  6 12 13 14 10 15 16
%t A318990 Select[Range[100],And[PrimeOmega[#]==2,Or[PrimePowerQ[#],Divisible@@Reverse[PrimePi/@FactorInteger[#][[All,1]]]]]&]
%o A318990 (PARI) ok(n)={my(f=factor(n)); bigomega(f)==2 && (#f~==1 || primepi(f[2,1]) % primepi(f[1,1]) == 0)} \\ _Andrew Howroyd_, Oct 26 2018
%Y A318990 A subset of A001358 (semiprimes), squarefree A006881.
%Y A318990 The squarefree version is A339005.
%Y A318990 The quotient is A358103 = A358104 / A358105.
%Y A318990 A000040 lists the primes.
%Y A318990 A001222 counts prime indices, distinct A001221.
%Y A318990 A003963 multiplies together prime indices.
%Y A318990 A056239 adds up prime indices.
%Y A318990 A358192/A358193 gives quotients of semiprime indices.
%Y A318990 Cf. A000837, A300912, A318991, A318992, A318993.
%Y A318990 Cf. A000720, A027751, A032741, A215366, A296150.
%Y A318990 Cf. A289508, A289509, A358106.
%K A318990 nonn
%O A318990 1,1
%A A318990 _Gus Wiseman_, Sep 06 2018