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Numbers that are not the product of two distinct composite numbers (A002808).
2

%I #14 Sep 08 2022 08:46:19

%S 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,25,26,27,

%T 28,29,30,31,33,34,35,37,38,39,41,42,43,44,45,46,47,49,50,51,52,53,55,

%U 57,58,59,61,62,63,65,66,67,68,69,70,71,73,74,75,76,77

%N Numbers that are not the product of two distinct composite numbers (A002808).

%e Number 24 is not in the sequence because 24 = 4 * 6.

%o (Magma) [n: n in [1..1000] | (#PrimeDivisors(n) eq 1 and NumberOfDivisors(n) lt 6) or (#PrimeDivisors(n) eq 2 and NumberOfDivisors(n) lt 8) or (#PrimeDivisors(n) ge 3 and NumberOfDivisors(n) lt 12) ]

%Y Complement of A289554.

%Y Cf. A246716 (with two distinct primes), A008578 (with two distinct nonprimes), A289553 (with two distinct noncomposites).

%K nonn

%O 1,2

%A _Jaroslav Krizek_, Jul 07 2017