code: def _logpdf(self, x, a, b):
      return a * log(b) - gammaln(a) - (a + 1) * log(x) - b / x
  
file_path: statsmodels/statsmodels/sandbox/examples/bayesprior.py
imports: import numpy
  from matplotlib import pyplot
  from scipy import stats, integrate
  from scipy.stats import rv_continuous
  from scipy.special import gammaln, gammaincinv, gammainc
  from numpy import log, exp
label_desc: Use image node_id 2 for calling the igamma_gen obj's underlying member method code with example usage: obj._logpdf(x, a, b) and returns: unknown