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 2020-09-01, 08:43 #1 enzocreti   Mar 2018 17×31 Posts ...factorial(n) plus/minus 1 is semiprime Is there an Oeis sequence dedicated to numbers n such that n!+1 and n!-1 are both semiprime? An example is 20970 Infact 20970!+1 is semiprime and also 20970!-1 is semi prime No the example is wrong Last fiddled with by enzocreti on 2020-09-01 at 08:46
 2020-09-01, 17:13 #2 Dylan14     "Dylan" Mar 2017 2·172 Posts This would be the intersection of 078778 (n such that n!+1 is semiprime) and 078781 (n such that n!-1 is semiprime). There is no separate sequence (yet, as far as I can tell).
 2020-09-02, 03:32 #3 CRGreathouse     Aug 2006 32·5·7·19 Posts The first few terms are: 5, 8, 10, 13, 20, 24, 26, 34, 59, 392. 392! - 1 = 78958785391722157 * P832. 392! + 1 = 32749 * P844.
2020-09-02, 16:35   #4
sweety439

Nov 2016

2,819 Posts

Quote:
 Originally Posted by enzocreti Is there an Oeis sequence dedicated to numbers n such that n!+1 and n!-1 are both semiprime? An example is 20970 Infact 20970!+1 is semiprime and also 20970!-1 is semi prime No the example is wrong
Neither 20970!+1 nor 20970!-1 is semiprime, 20970!+1 = 7778806327 * C81510, and 20970!-1 = 4156001 * C81513

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