--- In

primenumbers@yahoogroups.com, "Kevin Acres" <research@...> wrote:

> > Definition: Let F(n) = ((5^n-9)/4)^2-5 for integer n > 0.

> > Exercise 2: For odd n > 1, prove that F(n)/4 is composite.

>

> I did work out an algorithm to derive one divisor of F(n)/4 for odd n, but

> that's probably a long way from proving it composite.

What is needed is a formula: F(n)=L(n)*M(n). Then it suffices

to show that each of L(n) and M(n) has an odd prime divisor

for odd n > 1. Here is a clue:

4*F(11) = 24414063^2 - 15625^2

David