Introduction to Category Theory/Equalizers

Example

 * $$ f(x,y) = x^2 + y^2, g(x,y) = 1\;$$

Equalizer is $$\{(x,y) \in \mathbb{R}^2\;|\; x^2 + y^2=1\}$$ plus projection map.

Related Resources

 * Equaliser (mathematics)
 * Coequalizer