write this function p(ρ,τ)=a(τ)e^(-ρb(τ)) in matlab

You can define the function p(ρ,τ) as follows in MATLAB:

main.m
function result = p(rho,tau)
  a = @(t) 2*t.^2; % example function for 'a'
  b = @(t) sin(t); % example function for 'b'
  result = a(tau).*exp(-rho.*b(tau));
end
163 chars
6 lines

Here, a and b are example functions defined as function handles using anonymous functions in MATLAB. You can replace them with your own functions as per your requirement.

In the code above, we are defining result as the output of the function, which is obtained by multiplying the output of function a(tau) with the output of exp(-rho*b(tau)).

Note: In MATLAB, exp(x) function returns the exponential of x.

Now you can call this function p by passing values of rho and tau as arguments:

main.m
>> p(3,5)  % example call
ans =
   1.2977e-03
46 chars
4 lines

This call will return the value of p(ρ,τ) when ρ=3 and τ=5 (taken as example inputs).

Hope this helps!