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A053409
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Fibonacci numbers which are semiprimes.
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13
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21, 34, 55, 377, 4181, 17711, 121393, 1346269, 5702887, 165580141, 53316291173, 956722026041, 2504730781961, 308061521170129, 806515533049393, 14472334024676221, 1779979416004714189, 19740274219868223167, 573147844013817084101, 10284720757613717413913
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OFFSET
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1,1
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COMMENTS
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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
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LINKS
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FORMULA
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MATHEMATICA
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Select[Fibonacci[Range[150]], PrimeOmega[#]==2&] (* Harvey P. Dale, Jun 26 2020 *)
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PROG
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(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
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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