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 A091415 Numbers n such that n!*2^n - 1 is prime. 7
 2, 3, 4, 8, 13, 32, 41, 45, 59, 97, 107, 364, 421, 444, 1164, 1663, 3202, 4335, 4841, 13528, 22159, 38095, 50327, 72853 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Table of n, a(n) for n=1..24. Ken Davis, Status of Search for Multifactorial Primes. Ken Davis, Results for n!2-1. FORMULA a(n) = A007749(n+1)/2. - Alexander Adamchuk, Sep 23 2006 EXAMPLE a(1)=2 because 2!*2^2 - 1 = 7 is prime a(2)=3 because 3!*2^3 - 1 = 47 is prime MATHEMATICA For[n=1, n<1000, n++, If[PrimeQ[2^n*n!-1], Print[n]]] (Steinerberger) PROG (PARI) f(n)=n!*2^n -1; for (i=1, 363, if(isprime(f(i)), print(i))) CROSSREFS A093173 gives the corresponding primes. Cf. A007749, A117141, A092075. Sequence in context: A186272 A361722 A092075 * A166342 A361502 A091816 Adjacent sequences: A091412 A091413 A091414 * A091416 A091417 A091418 KEYWORD hard,more,nonn AUTHOR Mohammed Bouayoun (bouyao(AT)wanadoo.fr), Mar 02 2004 EXTENSIONS a(12)-a(14) from Stefan Steinerberger, Feb 05 2006 a(15) from Mohammed Bouayoun (Mohammed.Bouayoun(AT)yahoo.fr), Apr 13 2006 More terms from Alexander Adamchuk, Sep 23 2006 Corrected and extended by Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 03 2008 Terms a(22)..a(24) (using A007749) from Joerg Arndt, Apr 22 2016 STATUS approved

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Last modified September 27 15:23 EDT 2023. Contains 365711 sequences. (Running on oeis4.)