47

This number is a prime.

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The forty-seventh Fibonacci and Lucas numbers are both prime and are respectively 2971215073 and 6643838879. Forty-seven represents the largest subscript less than one hundred for which this is true. Among the first one hundred Fibonacci numbers and Lucas numbers, both are prime for the subscripts 4, 5, 7, 11, 13, 17, and 47. [Schiffman]

Submitted: 2010-11-01 13:48:00; Last Modified: 2010-11-14 00:21:41.