Question

There are n + 1 independent variables, namely t (the time
parameter) and x1, x2, . . . , xn (the space parameters), and
one dependent variable u = u(t, x1, x2, . . . , xn), subject to:
@u
@t
= k
Xn
i=1
@2u
(@xi )2...this is The n-dimensional heat equation.then why we cal n+1indipendent variable?we cal n indipendent variable?