Find the closest Fibonacci Number
Python 2, 43 bytes



f=lambda n,a=0,b=1:a*(2*n&lt;a+b)or f(n,b,a+b)


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Iterates through pairs of consecutive Fibonacci numbers (a,b) until it reaches one where the input n is less than their midpoint (a+b)/2, then returns a.

Written as a program (47 bytes):

n=input()
a=b=1
while 2*n&gt;a+b:a,b=b,a+b
print a


Same length:

f=lambda n,a=0,b=1:b/2/n*(b-a)or f(n,b,a+b)