A252663 Numbers m such that 10^m - m is a semiprime.
1, 7, 9, 11, 15, 33, 77, 93, 107, 117, 143, 149, 177, 209, 221
Offset: 1
Examples
1 is in this sequence because 10^1-1 = 3*3 is semiprime. 9 is in this sequence because 10^9-9 = 67*14925373 and these two factors are prime.
Links
- factorbd.com, Status of 10^269-269.
Crossrefs
Cf. similar sequences listed in A252656.
Programs
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Magma
IsSemiprime:=func; [m: m in [1..80] | IsSemiprime(s) where s is 10^m-m];
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Mathematica
Select[Range[80], PrimeOmega[10^# - #]==2 &]
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PARI
is(m) = bigomega(10^m - m) == 2; \\ Jinyuan Wang, Jul 09 2019
Extensions
a(8) from Luke March, Jul 08 2015
a(9)-a(13) from Daniel Suteu, Jul 09 2019
a(14), a(15) from Hugo Pfoertner, Aug 04 2019
Comments