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

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A122118 Least prime factor of 2^n + 5^n. 1
2, 7, 29, 7, 641, 7, 29, 7, 17, 7, 29, 7, 641, 7, 29, 7, 97, 7, 29, 7, 641, 7, 29, 7, 17, 7, 29, 7, 641, 7, 29, 7, 193, 7, 29, 7, 73, 7, 29, 7, 17, 7, 29, 7, 641, 7, 29, 7, 97, 7, 29, 7, 641, 7, 29, 7, 17, 7, 29, 7, 641, 7, 29, 7, 274568286337, 7, 29, 7, 137, 7, 29, 7, 17, 7, 29, 7, 457 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,1

COMMENTS

a(n_odd)=7, a(n=2+4k,k=0,1,...)=29, a(64)=274568286337 is unusually large.

LINKS

Hans Havermann, Table of n, a(n) for n = 0..1023 (terms 0..319 from Antti Karttunen)

FORMULA

a(n) = A020639(A074600(n)). - Antti Karttunen, Nov 02 2018

MATHEMATICA

Table[FactorInteger[2^n+5^n][[1, 1]], {n, 0, 80}] (* or *) Riffle[Table[ FactorInteger[2^n+5^n][[1, 1]], {n, 0, 80, 2}], 7] (* The second program is faster *) (* Harvey P. Dale, Mar 02 2015 *)

PROG

(PARI) A122118(n) = { my(k=(2^n+5^n)); forprime(p=if(64==n, 274568286337, 2), k, if(!(k%p), return(p))); }; \\ Antti Karttunen, Nov 02 2018

CROSSREFS

Cf. A020639, A074600 (2^n + 5^n), A094475 (Primes of form 2^n + 5^n), A122119.

Cf. also A094473.

Sequence in context: A013181 A191478 A321181 * A059803 A076043 A180448

Adjacent sequences:  A122115 A122116 A122117 * A122119 A122120 A122121

KEYWORD

nonn

AUTHOR

Zak Seidov, Oct 19 2006

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 October 22 16:32 EDT 2019. Contains 328319 sequences. (Running on oeis4.)