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

1,1

Links

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

Example

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

Mathematica

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

Prog

(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

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.

Sequence in context: A394433 A054395 A142863 * A375182 A132435 A108631

Adjacent sequences: A318987 A318988 A318989 * A318991 A318992 A318993

Keyword

nonn

Author

Gus Wiseman, Sep 06 2018

Status

approved