How can I write the following function in Matlab?

please help me. i want this function to solve following optimization problem. can you help me?

Answer 1

[Walter Roberson]

function r = rho(r,c)
  r = c^2/6 .* ones(size(r));
  mask = abs(r) <= c;
  r(mask) = c^2/6 .* (1 - (1 - r(mask)./c).^2).^3);

[esmail chogoli]

thank you for your answer.but when i want to write this code in matlab give me an error for mask