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

4, 6, 9, 10, 14, 21, 22, 25, 26, 34, 38, 39, 46, 49, 57, 58, 62, 65, 74, 82, 86, 87, 94, 106, 111, 115, 118, 121, 122, 129, 133, 134, 142, 146, 158, 159, 166, 169, 178, 183, 185, 194, 202, 206, 213, 214, 218, 226, 235, 237, 254, 259, 262, 267, 274, 278, 289

Offset: 1

Author

Gus Wiseman, Sep 06 2018

Keywords

• nonn

Examples

			The sequence of all dividing pairs (columns) begins:
  1  1  2  1  1  2  1  3  1  1  1  2  1  4  2  1  1  3  1  1  1  2  1  1
  1  2  2  3  4  4  5  3  6  7  8  6  9  4  8 10 11  6 12 13 14 10 15 16
		

Links

• Andrew Howroyd, Table of n, a(n) for n = 1..10000

Crossrefs

A subset of A001358 (semiprimes), squarefree A006881.

The squarefree version is A339005.

The quotient is A358103 = A358104 / A358105.

A000040 lists the primes.

A001222 counts prime indices, distinct A001221.

A003963 multiplies together prime indices.

A056239 adds up prime indices.

A358192/A358193 gives quotients of semiprime indices.

Cf. A000837, A300912, A318991, A318992, A318993.

Cf. A000720, A027751, A032741, A215366, A296150.

Cf. A289508, A289509, A358106.

Programs

Mathematica

Select[Range[100],And[PrimeOmega[#]==2,Or[PrimePowerQ[#],Divisible@@Reverse[PrimePi/@FactorInteger[#][[All,1]]]]]&]

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