A185999 Automorphic semiprimes: semiprimes, sp, such that sp is the k-th semiprime and sp ends in k.

291, 24502749, 36627829, 3547310731, 4721984179, 461808766011

Offset

1,1

Comments

Analogy, this sequence is to semiprimes (A001358) as A046883 is to the primes (A000040) or as A035383 is to the squares (A000290) among many others.

Links

Table of n, a(n) for n=1..6.

Index entries for sequences related to automorphic numbers

Mathematica

nextSemiPrime[n_] := Block[{k = n + 1}, While[ Plus @@ Last /@ FactorInteger@ k != 2, k++]; k]; c = 1; k = 4; lst = {}; While[k < 8100000000, If[ Mod[k, 10^Floor[1 + Log10@ c]] == c, AppendTo[lst, k]; Print[{c, k}]]; c++; k = nextSemiPrime@ k]; lst
These terms can be crosschecked by: SemiPrimePi[n_] := Sum[ PrimePi[n/Prime@ i] -i + 1, {i, PrimePi@ Sqrt@ n}]

Crossrefs

Cf. A001358, A046883, A046883, A186000.

Sequence in context: A158254 A088892 A098248 * A048956 A043439 A236049

Adjacent sequences: A185996 A185997 A185998 * A186000 A186001 A186002

Keyword

hard,nonn,base

Author

Jonathan Vos Post & Robert G. Wilson v, Feb 24 2011

Extensions

a(6) from Donovan Johnson, Mar 03 2011

Status

approved