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A104876
Semiprimes of the form primorial(k) - 1.
2
209, 510509, 6469693229, 200560490129, 13082761331670029, 1922760350154212639069, 557940830126698960967415389, 40729680599249024150621323469, 2305567963945518424753102147331756069, 232862364358497360900063316880507363069
OFFSET
1,1
LINKS
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 *)
KEYWORD
easy,nonn
AUTHOR
Jonathan Vos Post, Mar 28 2005
EXTENSIONS
Entry revised by N. J. A. Sloane, Apr 01 2006
STATUS
approved