Crab (or quarter) is a c/4 diagonal spaceship that was found by Jason Summers in September 2000. It is the smallest known diagonal spaceship other than the glider and it consists of two gliders pulling a tagalong.
It was noticed by Nicolay Beluchenko in August 2005 that it can be converted into a small tubstretcher, as shown below. This tubstretcher is the basis for the one per generation pattern.
Due to the diagonal glide symmetry and by simple inspection, the spaceship can easily be made to stretch two tub wicks simultaneously, via a domino spark repeatedly adding units to a tub or long^N barge. Additional non-stretching variants are possible by adding tub-eating c/4 spaceships at the back of the chain(s), at any distance.
Martin Grant found a synthesis for crab with the help of Tanner Jacobi and Brett Berger on December 26, 2014. The cost has since been reduced several times, most recently to 14 gliders by Tanner Jacobi on August 16, 2020.
The crab as a tubstretcher.RLE: here
- 25P4H1V1.1 at Heinrich Koenig's Game of Life Object Catalogs