MathCastle
Groups and FieldsHardHomomorphisms and Isomorphisms

Homomorphisms and Isomorphisms · Hard problem

Define φ:Z12×Z18→Z12×Z18\varphi: \mathbb Z_{12} \times \mathbb Z_{18} \to \mathbb Z_{12} \times \mathbb Z_{18} by φ(a,b)=(4a mod 12, 9b mod 18)\varphi(a,b) = (4a \bmod 12,\ 9b \bmod 18). How many elements does ker⁡φ\ker\varphi have?