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A252795 Numbers m such that 10^m + m is a semiprime. 1
3, 7, 37, 43, 49, 51, 57, 127 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

a(9) >= 217. - Hugo Pfoertner, Aug 06 2019

LINKS

Table of n, a(n) for n=1..8.

factordb.com, Status of 10^217+217.

EXAMPLE

3 is in this sequence because 10^3+3 = 17*59 is semiprime.

7 is in this sequence because 10^7+7 = 941*10627 and these two factors are prime.

MATHEMATICA

Select[Range[70], PrimeOmega[10^# + #]==2 &]

PROG

(MAGMA) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [m: m in [1..70] | IsSemiprime(s) where s is 10^m+m];

(PARI) is(n)=bigomega(10^n + n)==2 \\ Anders Hellström, Aug 15 2015

CROSSREFS

Cf. similar sequences listed in A252788.

Cf. A089379, A252663.

Sequence in context: A269995 A281093 A199347 * A049493 A260380 A260382

Adjacent sequences:  A252792 A252793 A252794 * A252796 A252797 A252798

KEYWORD

nonn,more,hard

AUTHOR

Vincenzo Librandi, Dec 27 2014

EXTENSIONS

a(8) from Carl Schildkraut, Aug 15 2015

STATUS

approved

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Last modified July 8 19:20 EDT 2020. Contains 335524 sequences. (Running on oeis4.)