A252789 Numbers m such that 4^m + m is a semiprime.

7, 19, 39, 43, 87, 135, 147, 177, 223, 255, 403

Offset

1,1

Comments

From Kevin P. Thompson, Apr 26 2022: (Start)

a(12) >= 765.

795 and 949 are also terms in this sequence. (End)

Links

Table of n, a(n) for n=1..11.

FactorDB, Status of 4^765+765.

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

a(10)-a(11) from Kevin P. Thompson, Apr 26 2022

Status

approved