A247606 Number of non-semiprimes among "preprimes" of the n-th kind (defined in comment in A247395).

1, 7, 15, 36, 31, 62, 59, 111, 161, 113, 224, 175, 155, 258, 370, 358, 240, 436, 346, 297, 557, 504, 691, 806, 477, 367, 554, 489, 938, 1743, 786, 959, 725, 1526, 669, 1215, 1207, 1022, 1359, 1286, 958, 1947, 773, 1206, 1328, 3078, 2740, 1165, 915, 1459, 1787

Offset

1,2

Comments

One can prove that non-semiprimes we can find among preprimes of the n-th kind only with the smallest prime divisor 2,3,...,prime(n), where n=1 corresponds to A156759, n=2 corresponds to A247393, n=3 corresponds to A247394, etc. For example, for n=1, only among even numbers of A156759; for n=2 - only among even numbers and numbers with the smallest prime divisor 3 of A247393, etc. Thus, for every n>=1, among preprimes of the n-th kind almost all numbers are semiprimes.

Links

Table of n, a(n) for n=1..51.

Vladimir Shevelev, A classification of the positive integers over primes

Crossrefs

Cf. A156759, A247393, A247394, A247395, A247396, A247509, A247510, A247511.

Sequence in context: A174792 A063592 A159792 * A146837 A146044 A048694

Adjacent sequences: A247603 A247604 A247605 * A247607 A247608 A247609

Keyword

nonn

Author

Vladimir Shevelev, Sep 22 2014

Extensions

More terms from Peter J. C. Moses, Sep 22 2014

Status

approved