I noted that the 2-engine Cordership is endemic, so the answer was at most 100 cells, but then I found that 60P312 + Gabriel's p138 achieves 96 cells, and this is the smallest endemic pattern I know of:What is the smallest periodic pattern by number of cells that is endemic (exact evolutionary sequence is unique) to B3/S23? The pattern does not have to be indecomposable into smaller periodic patterns (i.e. it can be pseudo).
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x = 95, y = 42, rule = B3/S23
20b2o$20b2o5$18b2o$19bo$6bo12bo15bo$5bobo10bo15bobo$5bobo26bobo$6bo28b
o46b3o$81bo2bo$81bo2bo$81b2o4$6bo2bo81b3o$6b3o82bo2bo$2o38b2o52bo$2o
38b2o32b3o15b3o$33b3o38bo$32bo2bo38bo2bo$75b3o4$86b2o$84bo2bo$6bo28bo
48bo2bo$5bobo26bobo47b3o$5bobo15bo10bobo$6bo15bo12bo$22bo$22b2o5$20b2o
$20b2o!
It is trivial to derive a lower bound of 19 cells – there are 16 S2 and S3 transitions, but the S3a transition can only happen in a block.What is the smallest still life by number of cells whose cells exercise all S2 and S3 transitions, thus having a minrule containing S23 per se? Strict and pseudo should be considered separately.