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Old 2020-09-01, 08:43   #1
enzocreti
 
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Default ...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
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Old 2020-09-01, 17:13   #2
Dylan14
 
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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).
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Old 2020-09-02, 03:32   #3
CRGreathouse
 
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The first few terms are: 5, 8, 10, 13, 20, 24, 26, 34, 59, 392.

392! - 1 = 78958785391722157 * P832.
392! + 1 = 32749 * P844.
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Old 2020-09-02, 16:35   #4
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Quote:
Originally Posted by enzocreti View Post
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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