A242134 Number of semiprimes with n digits.

3, 31, 265, 2326, 20753, 186657, 1694289, 15522934, 143361278, 1332987907, 12466213899, 117166026836, 1105962031475, 10478814259427, 99613914990801, 949727475528388, 9078425620890083, 86983554771459735, 835176988544997091, 8034305823411044844, 77423350444120266442

Offset

1,1

Links

Table of n, a(n) for n=1..21.

Formula

a(n) = A066265(n) - A066265(n-1). - Jens Kruse Andersen, Aug 16 2014

Example

4, 6, and 9 are semiprimes with 1 digit. So a(1) = 3.

Prog

(PARI) vector(8, n, sum(k=10^(n-1), 10^n-1, bigomega(k)==2))

Crossrefs

Cf. A001358, A066265.

Sequence in context: A198852 A114849 A221899 * A221894 A198964 A212730

Adjacent sequences: A242131 A242132 A242133 * A242135 A242136 A242137

Keyword

nonn,hard,base

Author

Derek Orr, Aug 15 2014

Extensions

a(9)-a(18) from A066265, added by Jens Kruse Andersen, Aug 16 2014

a(19)-a(21) from A066265, added by Jon E. Schoenfield, Feb 10 2022

Status

approved