A104876 Semiprimes of the form primorial(k) - 1.

209, 510509, 6469693229, 200560490129, 13082761331670029, 1922760350154212639069, 557940830126698960967415389, 40729680599249024150621323469, 2305567963945518424753102147331756069, 232862364358497360900063316880507363069

Offset

1,1

Links

Table of n, a(n) for n=1..10.

Sebastian Martin Ruiz, A Result on Prime Numbers, Math. Gaz. 81, 269, 1997.

Eric Weisstein's World of Mathematics, Primorial.

Eric Weisstein's World of Mathematics, Semiprime.

Formula

n# - 1 iff semiprime. Equals {A002110(i) - 1} intersection {A001358(j)}.

Example

4# - 1 = 209 = 11 * 19.
7# - 1 = 510509 = 61 * 8369.
10# - 1 = 6469693229 = 79 * 81894851.

Mathematica

Bigomega[n_]:=Plus@@Last/@FactorInteger[n]; SemiprimeQ[n_]:=Bigomega[n]==2; Primorial[n_]:=Product[Prime[i], {i, n}]; Select[Table[Primorial[n]-1, {n, 30}], SemiprimeQ] (* Ray Chandler, Mar 28 2005 *)

Crossrefs

Cf. A001358, A002110, A034386, A005234, A014545, A018239, A006794, A057704, A057705, A104877.

Sequence in context: A071379 A283153 A125549 * A349944 A259916 A050516

Adjacent sequences: A104873 A104874 A104875 * A104877 A104878 A104879

Keyword

easy,nonn

Author

Jonathan Vos Post, Mar 28 2005

Extensions

Entry revised by N. J. A. Sloane, Apr 01 2006

Status

approved