Subscript indices must either be real positive integers or logicals.

I am attempting to write a code for 6th order Newton's Interpolation. I am getting the error stated above and I'm not quite sure how to fix it. Here is my code so far.

% data
x=[0, 1.8,5, 6, 8.2, 9.2, 12];
y=[26, 16.415, 5.375, 3.5, 2.015, 2.54, 8];
X=input('x value=')
% divided differences
f1= (y(2)-y(1))/(x(2)-x(1));
f2= (y(3)-y(2))/(x(3)-x(2));
f3= (y(4)-y(3))/(x(4)-x(3));
f4= (y(5)-y(4))/(x(5)-x(4));
f5= (y(6)-y(5))/(x(6)-x(5));
f6= (y(7)-y(6))/(x(7)-x(6));
b0= y(1);
b1= f1;
b2= (f2-f1)/(x(3)-x(1));
b3= ((f3-f2/x(3)-x(2))-b2)/(x(4)-x(1));
b4=(((f4-f3)/x(4)-x(3))-b3)/(x(5)-x(1));
b5=(((f5-f4)/x(6)-x(5))-b4)/(x(6)-x(1));
b6=(((f6-f5)/x(7)-x(6))-b5)/(x(7)-x(1));
% Formula
F=b0+b1*(X-x(1))+b2*(X-x(1))*(X-x(2))+b3(X-x(1))*(X-x(2))*(X-x(3))+b4*(X-x(1))*(X-x(2))*(X-x(3))*(X-x(4))+b5*(X-x(1))*(X-x(2))*(X-x(3))*(X-x(4))*(X-x(5))+b6*(X-x(1))*(X-x(2))*(X-x(3))*(X-x(4))*(X-x(5))*(X-x(6));
answer=F

Answer 1

[Alexandra Harkai]

F= ... b3(X-x(1)) ...

A '*' is missing:

F= ... b3*(X-x(1))...