Title: Homological subsets of Spec,
Abstract: We investigate homological subsets of the prime spectrum of a ring, defined
by the help of the Ext-family $\{\Ext^i_R(-,R)\}$. We extend Grothendieck's
calculation of $\dim(\Ext^g_R(M,R))$. We compute support of $\Ext^i_R(M,R)$ in
many cases. Also, we answer a low-dimensional case of a problem posed by
Vasconcelos on the finiteness of associated prime ideals of
$\{\Ext^i_R(M,R)\}$. An application is given.