question: public static double logpoissonPDFm1 ( double x_plus_1 , double lambda ) { if ( Double . isInfinite ( lambda ) ) { return Double . NEGATIVE_INFINITY ; } if ( x_plus_1 > 1 ) { return rawLogProbability ( x_plus_1 - 1 , lambda ) ; } if ( lambda > Math . abs ( x_plus_1 - 1 ) * MathUtil . LOG2 * Double . MAX_EXPONENT / 1e-14 ) { return - lambda - GammaDistribution . logGamma ( x_plus_1 ) ; } else { return rawLogProbability ( x_plus_1 , lambda ) + Math . log ( x_plus_1 / lambda ) ; } }
target:  Compute the poisson distribution PDF with an offset of + 1 log pdf(x_plus_1 - 1, lambda)