# Ulam Spiral — Primes Puzzle | Mathallery

Source: https://mathallery.com/games/ulam/

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# Ulam Spiral

Primes · 30 levels

Sweep the spiral sky until only the primes shine.

## How to play

- The numbers wind out on a square spiral. Pick a brush, then touch any multiple of it.
- Every multiple of the brush number (but not the brush number itself) fades to dust: brush 3 clears 6, 9, 12…
- Clear every non-prime within the stroke limit. Only the primes stay lit, and their diagonal lines glow.

Every non-prime up to N has a factor no bigger than √N. Up to 100 you only need 2, 3, 5 and 7. In a window of big numbers, check each brush: does it clear anything the smaller ones miss?

[Open Ulam Spiral](https://mathallery.com/#/play/ulam/1)

Teeter, Tint and Echo Heist are free starters. Other games unlock through earned game choices or permanent packs. Your progress stays in the browser; optional sign-in saves it to your website account.