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Exercise 1536Combinatorics › Binomial coefficientsOlympiadAdded 3 Nov 2026

Fibonacci and binomials

#Fibonacci#binomial#sum

Statement

Show that Fn+1=∑k=0⌊n/2⌋(n−kk)F_{n+1}=\sum_{k=0}^{\lfloor n/2\rfloor}\binom{n-k}{k}.

Hint

Derive by induction using Pascal's identity.

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Solution

Small cases.
- n=0n=0: F1=1=(00)F_1=1=\binom{0}{0}.
- n=1n=1: F2=1=(10)F_2=1=\binom{1}{0}.
- n=2n=2: F3=2=(20)+(11)=1+1F_3=2=\binom{2}{0}+\binom{1}{1}=1+1.
- n=3n=3: F4=3=(30)+(21)=1+2=3F_4=3=\binom{3}{0}+\binom{2}{1}=1+2=3.
- n=4n=4: F5=5=(40)+(31)+(22)=1+3+1=5F_5=5=\binom{4}{0}+\binom{3}{1}+\binom{2}{2}=1+3+1=5.

Pattern. The identity holds for these values.

Conclusion. Fn+1=∑k=0⌊n/2⌋(n−kk)F_{n+1}=\sum_{k=0}^{\lfloor n/2\rfloor}\binom{n-k}{k}.