login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A216378 n such that n*10^n + 1 is semiprime. 1
2, 5, 13, 28, 34, 36, 39 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

This is to A007647 as semiprimes A001358 is to primes A000040. The corresponding semiprimes are A216376 = {201, 500001, 130000000000001, 280000000000000000000000000001, ...}.

LINKS

Table of n, a(n) for n=1..7.

FORMULA

{n such that A216376(n) is in A001358}.

EXAMPLE

a(1) = 2 because 2 * 10^2 + 1 = 201 = 3 * 67.

a(2) = 5 because  5 * 10^5 + 1 = 500001 = 3 * 166667.

a(3) = 13 because 13*10^13 + 1 = 130000000000001 = 6529 * 19911165569.

a(4) = 28 because 28 * 10^28 + 1 = 29 * 9655172413793103448275862069.

MATHEMATICA

Select[Range[40], PrimeOmega[# 10^# + 1] == 2 &] (* Alonso del Arte, Sep 08 2012 *)

PROG

(MAGMA) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [1..70] | IsSemiprime(s) where s is n*10^n+1]; // Vincenzo Librandi, May 10 2014

CROSSREFS

Cf. A001358, A007647, A064748, A216376.

Cf. similar sequences listed in A242203.

Sequence in context: A126656 A272794 A026522 * A225690 A193044 A122491

Adjacent sequences:  A216375 A216376 A216377 * A216379 A216380 A216381

KEYWORD

nonn,more

AUTHOR

Jonathan Vos Post, Sep 06 2012

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified June 25 12:01 EDT 2019. Contains 324352 sequences. (Running on oeis4.)