1 #ifndef STAN_MATH_REV_FUN_LOG1M_HPP
2 #define STAN_MATH_REV_FUN_LOG1M_HPP
3 
4 #include <stan/math/rev/meta.hpp>
5 #include <stan/math/rev/core.hpp>
6 #include <stan/math/prim/fun/log1m.hpp>
7 
8 namespace stan {
9 namespace math {
10 
11 /**
12  * The log (1 - x) function for variables.
13  *
14  * The derivative is given by
15  *
16  * \f$\frac{d}{dx} \log (1 - x) = -\frac{1}{1 - x}\f$.
17  *
18  * @tparam T Arithmetic or a type inheriting from `EigenBase`.
19  * @param a The variable.
20  * @return The variable representing log of 1 minus the variable.
21  */
22 template <typename T, require_stan_scalar_or_eigen_t<T>* = nullptr>
log1m(const var_value<T> & a)23 inline auto log1m(const var_value<T>& a) {
24   return make_callback_var(log1m(a.val()), [a](auto& vi) mutable {
25     as_array_or_scalar(a.adj())
26         += as_array_or_scalar(vi.adj()) / (as_array_or_scalar(a.val()) - 1.0);
27   });
28 }
29 
30 }  // namespace math
31 }  // namespace stan
32 #endif
33