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A242336
Numbers k such that k*5^k-1 is semiprime.
1
1, 2, 6, 12, 15, 19, 20, 26, 50, 55, 66, 68, 96, 99, 150, 166, 228, 459
OFFSET
1,2
COMMENTS
The semiprimes of this form are: 4, 49, 93749, 2929687499, 457763671874, 362396240234374, 1907348632812499, 38743019104003906249, 4440892098500626161694526672363281249, 15265566588595902430824935436248779296874, ...
a(19) >= 534. - Daniel Suteu, Aug 05 2019
MAPLE
select(t -> (numtheory:-bigomega(t*5^t-1)=2), [$1..400]); # Robert Israel, Aug 18 2015
MATHEMATICA
Select[Range[400], PrimeOmega[# 5^# - 1]==2&]
PROG
(Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [1..400] | IsSemiprime(s) where s is n*5^n-1];
CROSSREFS
Cf. similar sequences listed in A242273.
Sequence in context: A152389 A114103 A334308 * A285312 A065154 A354757
KEYWORD
nonn,more
AUTHOR
Vincenzo Librandi, May 12 2014
EXTENSIONS
1 prepended by Carl Schildkraut, Aug 18 2015
a(13)-a(17) from Carl Schildkraut, Aug 18 2015
a(18) from Daniel Suteu, Aug 05 2019
STATUS
approved