combinatòria

[ n // k_{1}+k_{2}+k_{3} = n ] = ( n!/(k_{1}!·k_{2}!k_{3}!) )

(x+y+z)^{2} = x^{2}+y^{2}+z^{2}+2xy+2yz+2zx

(x+y+z)^{3} = ...

... x^{3}+y^{3}+z^{3}+3x^{2}y+3y^{2}x+3x^{2}z+3z^{2}x+3y^{2}z+3z^{2}y+6xyz

f(k_{1},k_{2}) = [ n // k_{1}+k_{2} = n ]·2^{(-n)}

sum[ f(k_{1},k_{2}) ] = 1

f(k_{1},k_{2},k_{3}) = [ n // k_{1}+k_{2}+k_{3} = n ]·3^{(-n)}

sum[ f(k_{1},k_{2},k_{3}) ] = 1