1 #ifndef STAN_MATH_FWD_SCAL_FUN_INV_SQUARE_HPP
2 #define STAN_MATH_FWD_SCAL_FUN_INV_SQUARE_HPP
3 
4 #include <stan/math/fwd/core.hpp>
5 
6 #include <stan/math/prim/scal/fun/square.hpp>
7 
8 namespace stan {
9 namespace math {
10 
11 template <typename T>
inv_square(const fvar<T> & x)12 inline fvar<T> inv_square(const fvar<T>& x) {
13   T square_x(square(x.val_));
14   return fvar<T>(1 / square_x, -2 * x.d_ / (square_x * x.val_));
15 }
16 }  // namespace math
17 }  // namespace stan
18 #endif
19