Let *y *be some function of variable * x*; moreover, it is not essential, how this function is given: by formula or by table or by any
other way. Only the fact of existence of this functional dependence is important. This fact is written as: *y* = *f *( *x *). The letter
*f* ( it is initial letter of Latin word “functio” – a function ) doesn’t mean any value, as well as letters **log**, **sin**, **tan** in the
functions *y *= log *x*, *y* = sin *x*, *y* = tan *x. *They say only about the certain functional
dependence *y* of *x*. The record *y* = *f* ( *x *) represents *any *functional
dependence. If two functional dependencies *y* of *x* and *z* of *t*
differ one from the other, then they are written using different letters, for
instance: *y* = *f* ( *x *) and *z* = *F* ( *t* ). If some
dependencies are the same, then they are written by the same letter *f *:* y* = *f* ( *x *) and *z* = *f* ( *t* ). If an
expression for functional dependence *y* = *f* ( * x *) is known, then it can be written using both of the designations
of function. For instance, *y *= sin *x* or *f *( *x* ) = sin *x*. Both shapes are
equivalent completely. Sometimes another form of functional dependence is used: *y* ( *x* ).
This means the same as *y* = *f* ( *x *).