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A252788 Numbers m such that 3^m + m is a semiprime. 8
1, 4, 7, 14, 16, 20, 22, 32, 38, 55, 80, 92, 188, 220, 296, 328, 370, 422, 452, 454 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

a(21) >= 500. - Hugo Pfoertner, Aug 03 2019

LINKS

Table of n, a(n) for n=1..20.

factordb.com, Status of 3^500+500.

EXAMPLE

1 is in this sequence because 3^1+1 = 2*2 is semiprime.

14 is in this sequence because 3^14+14 = 283*16901 and these two factors are prime.

MATHEMATICA

Select[Range[130], PrimeOmega[3^# + #]==2 &]

PROG

(MAGMA) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [m: m in [1..130] | IsSemiprime(s) where s is 3^m+m];

(PARI) first(m)=my(v=vector(m), r=1); for(i=1, m, while(bigomega(3^r + r)!=2, r++); v[i]=r; r++); v; \\ Anders Hellström, Aug 14 2015

CROSSREFS

Cf. numbers m such that k^m+m is a semiprime: A085745 (k=2), this sequence (k=3), A252789 (k=4), A252790 (k=5), A252791 (k=6), A252792 (k=7), A252793 (k=8), A252794 (k=9), A252795 (k=10).

Cf. A252656.

Sequence in context: A310851 A310852 A072031 * A310853 A310854 A310855

Adjacent sequences:  A252785 A252786 A252787 * A252789 A252790 A252791

KEYWORD

nonn,more

AUTHOR

Vincenzo Librandi, Dec 22 2014

EXTENSIONS

a(13)-a(16) from Luke March, Jul 18 2015

a(17)-a(20) from Carl Schildkraut, Aug 19 2015

STATUS

approved

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Last modified September 30 03:03 EDT 2020. Contains 337432 sequences. (Running on oeis4.)