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Combinatorics
Binomial coefficients
Evaluate C(n, k) — the number of ways to choose k of n.
Practise →Stars and bars
Count non-negative integer solutions of a sum.
Practise →Lattice paths
Count monotonic grid paths from corner to corner.
Practise →Inclusion–exclusion
Count numbers divisible by none of the given primes.
Practise →Strings with no two adjacent 1s
Count length-n binary strings avoiding "11" — a Fibonacci recursion.
Practise →