20200901, 08:43  #1 
Mar 2018
2·5·53 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 20200901 at 08:46 
20200902, 03:32  #3 
Aug 2006
3·1,993 Posts 
The first few terms are: 5, 8, 10, 13, 20, 24, 26, 34, 59, 392.
392!  1 = 78958785391722157 * P832. 392! + 1 = 32749 * P844. 
20200902, 16:35  #4 
"99(4^34019)99 palind"
Nov 2016
(P^81993)SZ base 36
2·1,531 Posts 
Neither 20970!+1 nor 20970!1 is semiprime, 20970!+1 = 7778806327 * C81510, and 20970!1 = 4156001 * C81513

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