Let f1,f2,f3,f4 be four polynomials with real coefficients, having the property that
f_1 (1) =f_2 (0), f_2 (1) =f_3 (0), f_3 (1) =f_4 (0), f_4 (1) =f_1 (0) .
Prove that there exists a polynomial f∈R[X,Y] such that
f(X,0)=f_1(X), f(1,Y) =f_2(Y) , f(1-X,1) =f_3(X), f(0,1-Y)=f_4(Y) . algebrapolynomiallinear combination