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

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A053409 Fibonacci numbers which are semiprimes. 12
21, 34, 55, 377, 4181, 17711, 121393, 1346269, 5702887, 165580141, 53316291173, 956722026041, 2504730781961, 308061521170129, 806515533049393, 14472334024676221, 1779979416004714189, 19740274219868223167, 573147844013817084101, 10284720757613717413913 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Subsequence of A006881, since the only square Fibonacci numbers are 1 and 144. - Charles R Greathouse IV, Sep 24 2012

Apart from a(1) = 21, all terms are of the form F(p), F(2p), or F(p^2) where F(n) is the n-th Fibonacci number. - Charles R Greathouse IV, Oct 06 2016

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 1..48

FORMULA

a(n) = A000045(A072381(n)).

MATHEMATICA

Select[Fibonacci@Range[120], Last/@FactorInteger[#]=={1, 1}&] (* Vladimir Joseph Stephan Orlovsky, Jan 29 2012 *)

PROG

(PARI) issemi(n)=bigomega(n)==2

list(lim)=my(v=List([21]), F, t); forprime(p=2, , F=fibonacci(p); if(F>lim, break); if(issemi(F), listput(v, F))); forprime(p=2, , F=fibonacci(p^2); if(F>lim, break); if(isprime(t=fibonacci(p)) && isprime(F/t), listput(v, F))); forprime(p=2, , F=fibonacci(2*p); if(F>lim, break); if(isprime(t=fibonacci(p)) && isprime(F/t), listput(v, F))); Set(v) \\ Charles R Greathouse IV, Oct 06 2016

CROSSREFS

Cf. A000045, A001358, A006881, A072381.

Column k=2 of A303216.

Sequence in context: A135595 A082483 A075735 * A253910 A144205 A261408

Adjacent sequences:  A053406 A053407 A053408 * A053410 A053411 A053412

KEYWORD

nonn

AUTHOR

G. L. Honaker, Jr., Jan 09 2000

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 July 20 01:15 EDT 2019. Contains 325168 sequences. (Running on oeis4.)