  The survey provides classification results for integrable one-field evolution
equations of orders 2, 3 and 5 with the constant separant. The classification
is based on necessary integrability conditions following from the existence of
the formal recursion operator for integrable equations. Recurrent formulas for
the whole infinite sequence of necessary conditions are presented for the first
time. The most of the classification statements can be found in papers by S.I.
Svinilupov and V.V. Sokolov but the proofs have never been published before.
The result concerning the fifth order equations is stronger than obtained
before.
