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 A050937 Nonprime Fibonacci numbers with a prime index. 23
 1, 4181, 1346269, 24157817, 165580141, 53316291173, 956722026041, 2504730781961, 44945570212853, 308061521170129, 806515533049393, 14472334024676221, 1779979416004714189, 83621143489848422977 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS A Fibonacci number with a composite index is divisible by the Fibonacci numbers indexed by the divisors of the index (e.g., F(12) is divisible by F(3), F(4), F(6)), which would suggest that Fibonacci numbers indexed by primes are also themselves primes. This sequence clearly shows that not to be the case. REFERENCES David Wells, The Penguin Dictionary of Curious and Interesting Numbers, entry 4181. LINKS Amiram Eldar, Table of n, a(n) for n = 1..620 Vladimir Drobot, On primes in the Fibonacci sequence, Fib. Quart. 38 (1) (2000) 71 EXAMPLE Fibonacci(2) = 1 is not prime, but its index 2 is prime. Fibonacci(19) = 4181 is a composite Fibonacci number, but its index 19 is prime. MAPLE for n from 1 to 200 do if isprime(n) and (not isprime( fibonacci(n))) then print( fibonacci(n)): fi: od: MATHEMATICA Select[Table[Fibonacci[Prime[n]], {n, 25}], Not[PrimeQ[#]] &] (* Alonso del Arte, Nov 22 2010 *) PROG (PARI) f(n) = forprime(x=2, n, p=fibonacci(x); if(!isprime(p), print1(p", "))) \\ Cino Hilliard, Feb 11 2004 CROSSREFS Cf. A038672 (indices). Sequence in context: A072322 A045728 A048593 * A135953 A290846 A236805 Adjacent sequences:  A050934 A050935 A050936 * A050938 A050939 A050940 KEYWORD nonn,easy AUTHOR Jud McCranie, Jan 01 2000 STATUS approved

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Last modified September 21 03:13 EDT 2020. Contains 337266 sequences. (Running on oeis4.)