§ Fundamental theorem of homological algebra [TODO ]

§ Fundamental theorem

§ Corollary: two projective resolutions are chain homotopy equivalent

§ Proof of existence of projective resolution

     ker e
        |
        |   e
        vP0 -> M -> 0
    ker e
    ^   |
    |   v  e
P1---   P0 -> M -> 0

§ Chain homotopy classes of chain maps