function ret=p3(x)
  ret = zeros(1,length(x));
  alpha = pi/4;
  d = 1;
  lambda = 10;
  for (k=1:length(x))
    if (abs(x(k)) < 2*pi*d*sin(alpha)/lambda)
      ret(k) = 1/(2*alpha*sqrt((4*(pi^2)*(d^2)/(lambda^2))-(x(k))^2));
    end
  end
  ret = ret.^3;