Let A be an infinite
dimensional (associative) algebra over a field F. It is shown that A has an infinite
dimensional commutative subalgebra C of one of the following types:
C is generated by one element
C2= {0}
C3= {0} and C is an ideal of an ideal of A
C is generated by mutually orthogonal idempotents
C is a field.
A necessary and sufficient condition (in terms of the quadratic forms over F) is obtained
for the validity of the statement: Every infinite dimensional nil algebra over F has an
infinite dimensional subalgebra B with B2= {0}. An ideal y(A) of an algebra A
(analogous to the F.C. subgroup in group theory) is defined and several properties of
it are obtained.