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

 

Logo

Please make a donation to keep the OEIS running. We are now in our 55th year. In the past year we added 12000 new sequences and reached 8000 citations (which often say "discovered thanks to the OEIS"). We need to raise money to hire someone to manage submissions, which would reduce the load on our editors and speed up editing.
Other ways to donate

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A242205 Numbers n such that n*5^n+1 is semiprime. 1
1, 2, 4, 16, 21, 40, 76, 113, 153, 288 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

The semiprimes of this form are 6, 51, 2501, 2441406250001, 10013580322265626, 363797880709171295166015625001, ...

a(11) >= 1006. - Hugo Pfoertner, Aug 05 2019

LINKS

Table of n, a(n) for n=1..10.

factordb.com, Status of 1006*5^1006+1.

MATHEMATICA

Select[Range[90], PrimeOmega[# 5^# + 1] == 2 &]

PROG

(MAGMA) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [1..90] | IsSemiprime(s) where s is n*5^n+1];

CROSSREFS

Cf. similar sequences listed in A242203.

Cf. A050916.

Sequence in context: A212202 A102545 A045521 * A076434 A192150 A049441

Adjacent sequences:  A242202 A242203 A242204 * A242206 A242207 A242208

KEYWORD

nonn,more,hard

AUTHOR

Vincenzo Librandi, May 10 2014

EXTENSIONS

a(8)-a(10) from Luke March, Aug 13 2015

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 December 10 23:29 EST 2019. Contains 329910 sequences. (Running on oeis4.)