1.Nouna function of the type
f
:
V
×
V
→
V
{\displaystyle f:V\times V\rightarrow V}
, or more generally, a function of the type
f
:
U
1
×
U
2
×
.
.
.
×
U
n
→
V
{\displaystyle f:U_{1}\times U_{2}\times ...\times U_{n}\rightarrow V}
in which exactly two of the sets Ui are equal to V.