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Exercise 1039Logic, sets and proofs › Proof techniques › InductionL1Added 3 Nov 2026

$n!$ vs $2^n$

#induction#factorial#exponential

Statement

Prove that n!>2nn!>2^n for all n≥4n\ge4.

Hint

Base case n=4n=4; inductive step: multiply by (n+1)>2(n+1)>2.

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Solution

Base case. n=4n=4: 4!=24>16=244!=24>16=2^4. ✓

Inductive step. Assume n!>2nn!>2^n with n≥4n\ge4. Then
(n+1)!=(n+1)⋅n!>(n+1)2n>2⋅2n=2n+1,(n+1)!=(n+1)\cdot n!>(n+1)2^n>2\cdot2^n=2^{n+1},
since n+1>2n+1>2.

Conclusion. n!>2nn!>2^n for all n≥4n\ge4.