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 A112861 Numbers k such that (2*k)!/(2*(k!)^2) - 1 is prime. 6
 2, 6, 10, 21, 45, 63, 306, 404, 437, 471, 646, 1174, 1192, 1334, 1975, 2397, 2410, 4305, 6111, 7852, 9488, 11120, 13304, 14408, 16075, 16238 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The next term is > 20000. - Vaclav Kotesovec, May 02 2021 LINKS Table of n, a(n) for n=1..26. MATHEMATICA Select[Range[10000], PrimeQ[(2 #)! / (2 (#!)^2) - 1 ] &] (* Vincenzo Librandi, Apr 10 2015 *) PROG (Magma) [n: n in [1..700] | IsPrime(Factorial(2*n) div (2*Factorial(n)^2)-1)]; // Vincenzo Librandi, Apr 10 2015 CROSSREFS Cf. A001700(n-1) = (2*n)!/(2*(n!)^2); A112862: primes of the form (2*n)!/(2*(n!)^2)-1; A112853: (2*n)!/n!-1 is prime; A112855: (2*n)!/n!+1 is prime; A066726: (2*n)!/(n!)^2-1 is prime; A066699: (2*n)!/(n!)^2+1 is prime; A112863: (2*n)!/(2*(n!)^2)+1 is prime. Sequence in context: A034450 A297185 A304991 * A180230 A186296 A140775 Adjacent sequences: A112858 A112859 A112860 * A112862 A112863 A112864 KEYWORD hard,nonn AUTHOR Hugo Pfoertner, Sep 30 2005 EXTENSIONS a(22)-a(26) from Vaclav Kotesovec, May 02 2021 STATUS approved

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Last modified September 29 18:47 EDT 2023. Contains 365775 sequences. (Running on oeis4.)