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A252789 Numbers m such that 4^m + m is a semiprime. 1
7, 19, 39, 43, 87, 135, 147, 177, 223 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Table of n, a(n) for n=1..9.

EXAMPLE

7 is in this sequence because 4^7+7 = 37*443 and these two factors are prime.

19 is in this sequence because 4^19+19 = 11*24988900633 and these two factors are prime.

MATHEMATICA

Select[Range[130], PrimeOmega[4^# + #]==2 &]

PROG

(MAGMA) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [m: m in [1..130] | IsSemiprime(s) where s is 4^m+m];

(PARI) main(m)=select(m->bigomega(4^m + m)==2, vector(m, i, i)); \\ Anders Hellström, Aug 14 2015

CROSSREFS

Cf. similar sequences listed in A252788.

Cf. A252657.

Sequence in context: A051937 A119327 A152728 * A099061 A078163 A108766

Adjacent sequences:  A252786 A252787 A252788 * A252790 A252791 A252792

KEYWORD

nonn,more

AUTHOR

Vincenzo Librandi, Dec 24 2014

EXTENSIONS

a(6)-a(9) from Carl Schildkraut, Aug 14 2015

STATUS

approved

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Last modified October 18 13:21 EDT 2021. Contains 348067 sequences. (Running on oeis4.)