[Math] Mnemonic for the fact that a right(left) adjoint functor preserves limits(colimits)

adjoint-functorscategory-theory

A right adjoint functor preserves limits.
Dually a left adjoint functor preserves colimits.
I often forget which is which.
Of course, you can look up a book on category theory or use internet.
But it's nice if there is a good mnemonic method to remember these facts.

Best Answer

I remember this (and related facts) as follows: A left adjoint $F$ is characterized by morphisms on $F(x)$, and a colimit is characterized by morphisms on it. Dually, a right adjoint $G$ is characterized by morphisms into $G$, and a limit is characterized by morphisms into it. So basically I just reprove it all the time, after all it is only one line:

$(\mathrm{colim}_i F(x_i),-) = \mathrm{lim}_i (F(x_i),-) = \mathrm{lim}_i (x_i,G(-))=(\mathrm{colim}_i x_i,G(-))=(F(\mathrm{colim}_i x_i),-)$