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A038108 Number of prime pairs {p,q}, such that pq < n^2. 1
0, 2, 6, 8, 13, 16, 22, 26, 34, 39, 48, 55, 62, 75, 82, 89, 103, 113, 126, 135, 149, 163, 179, 190, 202, 220, 236, 252, 270, 288, 304, 320, 340, 360, 381, 403, 425, 443, 462, 483, 508, 532, 556, 581, 604, 633, 655, 678, 709, 738, 761, 782 (list; graph; refs; listen; history; text; internal format)
OFFSET

2,2

COMMENTS

Number of semiprimes (A001358) < n^2. [Michel Marcus, Sep 02 2013]

LINKS

Table of n, a(n) for n=2..53.

EXAMPLE

a(3)=2 because only the prime pairs (2,2) and (2,3) form products < 9.

PROG

(PARI) a(n) = {sqn = n^2; idp = primepi(sqn\2); nbp = 0; for (i = 1, idp, p = prime(i); for (j = 1, i, if (p * prime(j) < sqn, nbp++); ); ); nbp; } \\ Michel Marcus, Sep 02 2013

CROSSREFS

Cf. A205726.

Sequence in context: A229056 A186703 A054248 * A294862 A087327 A266627

Adjacent sequences:  A038105 A038106 A038107 * A038109 A038110 A038111

KEYWORD

nonn

AUTHOR

Joe K. Crump (joecr(AT)carolina.rr.com)

STATUS

approved

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Last modified November 14 17:24 EST 2019. Contains 329126 sequences. (Running on oeis4.)