issue with hypergeometric1F1

[amarshankarc]

Define the probability mass function (PMF) function

beta_poisson_pmf<- function(x, alpha, beta, N) {

Beta function term

beta_term <- gamma(x + alpha) gamma(alpha + beta) / (gamma(alpha) gamma(x + alpha + beta))

Poisson term

poisson_term <- N^x / gamma(x + 1)

Confluent hypergeometric function term

hypergeo_term <- hypergeometric1F1(x + alpha, x + alpha + beta, -N)

Combine terms

result <- beta_term poisson_term hypergeo_term

return(result) } print(beta_poisson_pmf(1,2,3,10)) IAM GETTING BETA POISSON PROBABILITY MASS FUNCTION AS NEGATIVE AND ABOVE ONE WHICH IS NOT POSSIBLE

[merliseclyde]

Please note from the documentation that BAS::hypergeometric1F1 computes the log of the function by default.

hypergeometric1F1(a, b, c, laplace = FALSE, log = TRUE)

> BAS::hypergeometric1F1(1+2,1 + 2 + 3,-10)
[1] -3.467487
> BAS::hypergeometric1F1(1+2,1 + 2 + 3,-10, log=FALSE)
[1] 0.03119531