Результат выполнения программы:

Default[f]=10; Clear[f]f[x_.]:=x^2; f[5]f[]

f[x_]:=x; f[x_,y_]:=xy;         Clear[f]Definition[f]

Default[f]=10; ClearAll[f]f[x_.]:=x^2; f[5]f[]

Default[f]=10; Default[f]=.f[x_.]:=x^2; f[5]f[]

f[x:{_,_}]:=x[[1]]+x[[2]];{f[{1,2}],f[{1,2,3}],f[x]}

f[x__]:={x}^2;{f[],f[2],f[2,3]}

f[x:{_,_}..]:=Plus@@{x};f[{x1,x2},{y1,y2},{z1,z2}]f[{x1,x2},y,{z1,z2}]f[{x1,x2},{y1,y2},{{z1,z2},z3}]

f[x_:10]:=x^2; f[5]f[]

x=1; Dynamic[x^2]x=3;

Dynamic[x^2]x=3;

f[x_Integer]:=x^2;{f[2],f[1/2],f[x]}