Documentation

# `stats`::`fPDF`

Probability density function of Fisher's f-distribution (fratio distribution)

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## Syntax

```stats::fPDF(`a`, `b`)
```

## Description

`stats::fPDF(a, b)` returns a procedure representing the probability density function of Fisher's f-distribution with shape parameters a > 0, b > 0.

The procedure `f:=stats::fPDF(a, b)` can be called in the form `f(x)` with an arithmetical expression `x`. The return value of `f(x)` is either a floating-point number or a symbolic expression:

If x ≤ 0 can be decided, then `f(x)` returns 0 If x > 0 can be decided, then `f(x)` returns the value .

If x is a floating-point number and both a and b can be converted to positive floating-point numbers, then these values are returned as floating-point numbers. Otherwise, symbolic expressions are returned.

The function `f` reacts to properties of identifiers set via `assume`. If x is a symbolic expression with the property x ≤ 0 or x ≥ 0, the corresponding values are returned.

`f(- infinity )` and `f( infinity )` return 0.

`f(x)` returns the symbolic call ```stats::fPDF(a, b)(x)``` if neither x ≤ 0 nor x > 0 can be decided.

Numerical values for `a` and `b` are only accepted if they are real and positive.

## Environment Interactions

The function is sensitive to the environment variable `DIGITS` which determines the numerical working precision. It reacts to properties of identifiers set via `assume`.

## Examples

### Example 1

We evaluate the probability density function with a = 2 and b = 4 at various points:

```f := stats::fPDF(2, 4): f(-infinity), f(-PI), f(1/2), f(0.5), f(PI), f(infinity)```
` `
`delete f:`

### Example 2

If `x` is a symbolic object without properties, then it cannot be decided whether x ≥ 0 holds. A symbolic function call is returned:

`f := stats::fPDF(a, b): f(x)`
` `

With suitable properties, it can be decided whether x ≥ 0 holds. An explicit expression is returned:

`assume(0 <= x): f(x)`
` `
`unassume(x): delete f:`

### Example 3

We use symbolic arguments:

`f := stats::fPDF(a, b): f(x)`
` `

When numerical values are assigned to `a` and `b`, the function `f` starts to produce numerical values:

`a := 2: b := 1: f(3), f(3.0)`
` `
`delete f, a, b:`

## Parameters

 `a`, `b` The shape parameters: arithmetical expressions representing positive real values