ForHosting KIT · Developer Utilities

Stirling numbers of the second kind

Stirling numbers of the second kind answer a fundamental counting question in discrete mathematics: given n distinct objects, in how many ways can you partition them into exactly k non-empty unlabeled groups?

● BetaFree · in your browser

Run — free How it works & API

Use it from WebAPIEmailTelegramApp soon

The answer is written S(n,k) or sometimes {n choose k} with curly braces. These numbers appear whenever you group people into teams without naming the teams, split a set of keys into unlabeled buckets, count surjective functions up to a factorial factor, build Bell numbers as a sum over k, or grade homework that asks for set partitions rather than permutations. This capability accepts non-negative integers n (the number of items) and k (the number of non-empty subsets) and returns the exact S(n,k) as a decimal string computed with BigInt dynamic programming, so values far beyond ordinary safe-integer limits stay correct digit for digit. Inputs with a negative k or with k greater than n are rejected with a clear validation error instead of a silent zero or a wrong answer. The free browser tool and the paid API share one pure function, so classroom demos and automated pipelines never diverge on the same pair.

How to use it

Enter your values in the form above. The tool checks them before calculating and shows the result on the same page.

Check your inputs

Use the labels and units shown next to each field. If something is missing or outside the allowed range, the page points to the field to fix.

Use it again or automate it

Use the browser tool for individual checks and the API when you need the same capability in an automated workflow.

What you can do with it

Get an answer now

Enter one set of values and see the result without building a spreadsheet or script.

Compare scenarios

Change one value at a time and rerun the calculation to understand what affects the result.

Automate repeated work

Use the API when the same calculation needs to run inside your product or workflow.

FAQ

How do I use this capability?

Complete the fields above and run it on this page. The form highlights anything that needs attention.

For developers — API access

Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.

API endpoint

POSThttps://api.kit.forhosting.com/combo/stirling-second

Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.

Call it from your stack

curl -X POST https://api.kit.forhosting.com/combo/stirling-second \
  -H "Authorization: Bearer $KIT_KEY" \
  -H "Content-Type: application/json" \
  -d '{"n":5,"k":2}'

const res = await fetch("https://api.kit.forhosting.com/combo/stirling-second", {
  method: "POST",
  headers: {
    "Authorization": `Bearer ${process.env.KIT_KEY}`,
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    "n": 5,
    "k": 2
  })
});
const { task_id } = await res.json();

import os, requests

res = requests.post(
    "https://api.kit.forhosting.com/combo/stirling-second",
    headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
    json={
    "n": 5,
    "k": 2
},
)
task_id = res.json()["task_id"]

<?php
$res = file_get_contents("https://api.kit.forhosting.com/combo/stirling-second", false, stream_context_create([
  "http" => [
    "method"  => "POST",
    "header"  => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
    "content" => '{"n":5,"k":2}',
  ],
]));
$task = json_decode($res, true);

body := bytes.NewBufferString(`{"n":5,"k":2}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/combo/stirling-second", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)

Example request

{
  "n": 5,
  "k": 2
}

Example response

{
  "task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
  "type": "combo.stirling_second",
  "status": "queued",
  "_links": {
    "result": "/tasks/tsk_…/result"
  }
}

The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.

Pricing

Per request$0.002

Published price — no tokens, no invented credits. A failed task is never charged.

Limits

`min_n`	0
`max_n`	1000
`min_k`	0
`max_k`	1000

Errors

HTTP	Code	Meaning
`401`	`unauthorized`	Missing or invalid API key.
`402`	`insufficient_balance`	Your balance doesn't cover the task price.
`404`	`unknown_type`	That task type doesn't exist.
`429`	`rate_limited`	Too many requests. Use the webhook instead of polling.

Read the full KIT documentation →

Run — free