Are there infinitely many terms in Fibonacci sequence divisible by ?
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First terms of Fibonacci sequence are: 1 , 1 , 2 , 3 , 5 , 8 , 1 3 . So there is at least one. Next we take Fibonacci sequence modulo 1 3 . If there are consequtive terms of the form a , 0 (and there are 8 , 1 3 ) then next terms are: a , 0 , a , a , 2 a , 3 a , 5 a , 8 a , 1 3 a but 1 3 a = 0 modulo 1 3 and since 8 a , 0 is of the form b , 0 ( b here is just some value e.g. 8 a ) we obtain by induction that there are infinitely many terms divisible by 1 3 .