Few things -
Indexing in MATLAB starts with 1, not 0, thus these lines will lead to an error
t(i)=(v(i)-v(0))/a;
x(i)=v(0)*t(i)+(1/2)*a;
If you want to refer it the first element, use v(1).
Secondly, you can vectorize some parts of your code
%Assuming a and amax are a scalar constant
h=0.001;
%define a value as a variable if you have to use it many times
len=length(v);
%Rmax=zeros(1,len);
%t=zeros(1,len);
%x=zeros(1,len);
%Note that, as the code uses another variable to directly define
%t, x and Rmax, I have chosen not to preallocate them
%Assuming you want to refer to the first element
t=(v-v(1))/a;
x=v(1)*t+(1/2)*a;
Rmax=(v.^2)/amax;
% ^
%Since v is an array, it requires element-wise operation
However, it's not clear what you want to achieve with B and subsequently Rt.
B(x)=(1-x)^3*P0+3*x*(1-x)^2*P1+3*x^2*(1-x)*P2+t^3*P3;
Rt=(1+(diff(B)/h)^2)^(3/2)/(diff(B,2)/h);
Is B supposed to be a polynomial in x? If not then specify what it is supposed to be.
Are P0, P1, P2 and P3 supposed to be unknown whose value you have to find? If they are unknowns, how is one supposed to find the value of Rt to compare with Rmax?
And to which value of Rmax do you want to compare? As Rmax is an array.
