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§ Mnemonic for Hom-tensor and Left-right Adjoints

created 2021-03-29
  • Remember the phrase tensor-hom adjunction, thus tensor is left adjoint.
  • Remember that the type of an adjunction is (f x -> y) -> (x -> g y) and here, f is left adjoint, g is right adjoint. Then see that currying is ((p, x) -> y) ->(x -> (p -> y)). Thus tensor is left adjoint, hom is right adjoint.
  • Remember that RAPL (right adjoints preserve limits); Then recall that tensoring a direct limit (a colimit) preserves the tensor, as a colimit retains torsion (example: prufer group has torsion, its components also have torsion; tensor can detect this by tensoring with Q\mathbb QQ). On the other hand, tensoring of an inverse-limit (a limit) is not preserved: think of p-adics. Each of the components have torsion, but the p-adics do not. Thus, tensor DOES NOT preserve limits (inverse limits). And so, tensor CANNOT be right adjoint; tensor must be left adjoint.
  • Since tensor is right exact, because it kills stuff, and could this destroy injectivity, it is left adjoint.
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