Stan Math Library  2.15.0
reverse mode automatic differentiation
inc_beta_ddb.hpp
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1 #ifndef STAN_MATH_PRIM_SCAL_FUN_INC_BETA_DDB_HPP
2 #define STAN_MATH_PRIM_SCAL_FUN_INC_BETA_DDB_HPP
3 
7 #include <cmath>
8 
9 namespace stan {
10  namespace math {
11 
12  template <typename T>
13  T inc_beta_dda(T a, T b, T z,
14  T digamma_a, T digamma_ab);
15 
38  template <typename T>
39  T inc_beta_ddb(T a, T b, T z,
40  T digamma_b, T digamma_ab) {
41  using std::log;
42 
43  if (b > a)
44  if ((0.1 < z && z <= 0.75 && b > 500)
45  || (0.01 < z && z <= 0.1 && b > 2500)
46  || (0.001 < z && z <= 0.01 && b > 1e5))
47  return -inc_beta_dda(b, a, 1 - z, digamma_b, digamma_ab);
48 
49  if ((z > 0.75 && a < 500)
50  || (z > 0.9 && a < 2500)
51  || (z > 0.99 && a < 1e5)
52  || (z > 0.999))
53  return -inc_beta_dda(b, a, 1 - z, digamma_b, digamma_ab);
54 
55  double threshold = 1e-10;
56 
57  // Common prefactor to regularize numerator and denomentator
58  T prefactor = (a + 1) / (a + b);
59  prefactor = prefactor * prefactor * prefactor;
60 
61  T sum_numer = digamma_ab * prefactor;
62  T sum_denom = prefactor;
63 
64  T summand = prefactor * z * (a + b) / (a + 1);
65 
66  T k = 1;
67  digamma_ab += 1.0 / (a + b);
68 
69  while (fabs(summand) > threshold) {
70  sum_numer += digamma_ab * summand;
71  sum_denom += summand;
72 
73  summand *= (1 + (a + b) / k) * (1 + k) / (1 + (a + 1) / k);
74  digamma_ab += 1.0 / (a + b + k);
75  ++k;
76  summand *= z / k;
77 
78  if (k > 1e5)
79  domain_error("inc_beta_ddb",
80  "did not converge within 100000 iterations", "", "");
81  }
82 
83  return inc_beta(a, b, z)
84  * (log(1 - z) - digamma_b + sum_numer / sum_denom);
85  }
86 
87  }
88 }
89 #endif
fvar< T > fabs(const fvar< T > &x)
Definition: fabs.hpp:15
T inc_beta_dda(T a, T b, T z, T digamma_a, T digamma_ab)
Returns the partial derivative of the regularized incomplete beta function, I_{z}(a, b) with respect to a.
fvar< T > log(const fvar< T > &x)
Definition: log.hpp:14
T inc_beta_ddb(T a, T b, T z, T digamma_b, T digamma_ab)
Returns the partial derivative of the regularized incomplete beta function, I_{z}(a, b) with respect to b.
fvar< T > inc_beta(const fvar< T > &a, const fvar< T > &b, const fvar< T > &x)
Definition: inc_beta.hpp:19
void domain_error(const char *function, const char *name, const T &y, const char *msg1, const char *msg2)
Throw a domain error with a consistently formatted message.
double e()
Return the base of the natural logarithm.
Definition: constants.hpp:94

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