inverse.gamma.pdf <- function( lambda, alpha, beta ) {

  if ( sum( c( lambda, alpha, beta ) < 0 ) > 0 ) {

    print( 'illegal inputs' )

    return( NULL )

    break

  }

  else {

    pdf <- ( beta^alpha / gamma( alpha ) ) * lambda^( - ( alpha + 1 ) ) *
      exp( - beta / lambda )

    return( pdf )

  }

}