---
epic: 6
story: 6.3
title: "Factorial & Combinatorics"
status: draft
---

## Epic 6 — Advanced Math & Functions
**Goal:** Scientific, financial, and power-user math.

### Story 6.3: Factorial & Combinatorics

As a **CalcPad user**,
I want to compute factorials, permutations, and combinations,
So that I can solve probability, statistics, and combinatorics problems.

**Acceptance Criteria:**

**Given** the user writes `10!`
**When** the engine evaluates
**Then** the result is `3628800`

**Given** the user writes `0!`
**When** the engine evaluates
**Then** the result is `1` (by mathematical convention)

**Given** the user writes `nPr(10, 3)`
**When** the engine evaluates
**Then** the result is `720` (10! / 7!)

**Given** the user writes `nCr(10, 3)`
**When** the engine evaluates
**Then** the result is `120` (10! / (3! * 7!))

**Given** the user writes `100!`
**When** the engine evaluates
**Then** the result is computed using arbitrary precision arithmetic and is exact (not a floating-point approximation)
**And** the full integer result is displayed (or a scrollable/expandable representation for very large numbers)

**Given** the user writes `(-3)!`
**When** the engine evaluates
**Then** an error is displayed (factorial of negative integers is undefined)

**Given** the user writes `3.5!`
**When** the engine evaluates
**Then** either the gamma function is used to compute `Gamma(4.5)` approximately `11.6317`, or an error indicates that only non-negative integer factorials are supported

**Given** the user writes `nCr(5, 7)` where k > n
**When** the engine evaluates
**Then** the result is `0` (by combinatorial convention)