Sum the First n Even Fibonacci Numbers

c=2+5**.5
lambda n:(7-c)*c**n//20

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Magic formula!

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This uses the same recurrence formula as my MATL answer.

f=x=>x>1&&4*f(x-1)+f(x-2)+2

Recursion to the rescue! This uses one of the formulas on the OEIS page:

f(n < 1) = 0, f(n) = 4*a(n+1)+a(n)+2

(but shifted by one because the challenge shifts it by one)