Primer
From LifeWiki
Primer  
Pattern type  Miscellaneous  

Number of cells  2953  
Bounding box  440×294  
Discovered by  Dean Hickerson  
Year of discovery  1991  

Primer is a pattern that was constructed by Dean Hickerson on November 1, 1991 that produces a stream of lightweight spaceships representing prime numbers. N is prime if and only if a lightweight spaceship escapes to the left of the pentadecathlon at the bottomleft corner of the pattern at generation 120N+100.^{[1]}^{[2]} It was the first pattern created that computes prime numbers, though others have since been constructed using the same ideas (see glider gunless primer).
It works by using gliders to emulate a prime number sieve. Lightweight spaceships that move westward are deleted by gliders that represent positive integers if the lightweight spaceship represents a multiple of that number.
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References
 ↑ PRIMES.LIF from Alan Hensel's lifep.zip pattern collection. Accessed on July 28, 2009.
 ↑ fourprimers.rle.gz from Golly's builtin pattern catalogue. Accessed on July 28, 2009.