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PROJECT EULER · #1007

Alternating Difference

Statement only · UnsolvedOriginal problem ↗

The Fibonacci sequence is defined by F0=0,F1=1 and Fk=Fk1+Fk2 for k2.

Write down the numbers F0,F1,,Fn in a row and separate them with n minus signs .
We want to add n pairs of parentheses () to form a valid expression such that each pair of parentheses contains exactly one minus sign, not counting those contained in its subparentheses.

For example, when n=3, we can form five different expressions in this way:

(((F0F1)F2)F3)=4((F0(F1F2))F3)=2((F0F1)(F2F3))=0(F0((F1F2)F3))=2(F0(F1(F2F3)))=2

The sum of the values of these expressions is equal to 6.

Let A(n) be the sum of the values of all different expressions that can be obtained in this way.
Thus A(3)=6. Moreover, A(10)=177666 and A(100)71792794mod(109+9).

Find A(107)mod(109+9).

Write-up coming later

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