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A242134 Number of semiprimes with n digits. 0
3, 31, 265, 2326, 20753, 186657, 1694289, 15522934, 143361278, 1332987907, 12466213899, 117166026836, 1105962031475, 10478814259427, 99613914990801, 949727475528388, 9078425620890083, 86983554771459735 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Table of n, a(n) for n=1..18.

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

STATUS

approved

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Last modified May 6 11:12 EDT 2021. Contains 343581 sequences. (Running on oeis4.)