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 A104876 Semiprimes of the form primorial(k) - 1. 1
 209, 510509, 6469693229, 200560490129, 13082761331670029, 1922760350154212639069, 557940830126698960967415389, 40729680599249024150621323469, 2305567963945518424753102147331756069 (list; graph; refs; listen; history; text; internal format)
 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 *) CROSSREFS Cf. A001358, A002110, A034386, A005234, A014545, A018239, A006794, A057704, A057705, A104877. Sequence in context: A071379 A283153 A125549 * A259916 A050516 A100670 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

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Last modified March 23 14:17 EDT 2019. Contains 321431 sequences. (Running on oeis4.)