In the book "subsystems of second order arithmetic", page 68, Simpson claimed that the set of all codes of finite sequences (denoted by "Seq") exists by sigma 0-0 comprehension. I tried to write a sigma 0-0 formula like A(t) to express that t is code of a finite sequence, but I failed. The only formula I could write was the formula that using existential set quantifier. Can anybody help me with this?
2026-03-25 20:13:52.1774469632
The set of all finite sequences in RCA0
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