Blog archive
Sorted newest first — every guide ends with practice problems and full solutions.
If n+1 pigeons sleep in n holes, one hole is crowded — and that single sentence wins a shocking number of olympiad problems. Here is how to spot the pigeons hidden inside a statement.
When a problem asks whether a configuration can ever be reached, stop simulating and start looking for a quantity that never changes — or always changes the same way.
Combinatorics is not enumeration. It is finding a structure that turns a messy count into a clean product, a symmetry, or two counts of the same thing.
