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I'm trying to prove something in Fitch and i'm stuck on one step, i have:

1.  Cube(a) <-> a = a

and i want to derive 2. Cube(a) from that.

I know it's possible because i can use Ana Con on 2. and selecting 1. as premise and it says it's valid.
Is there anyone who can tell me how to do this without using Ana Con?

Aerus
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  • http://cstheory.stackexchange.com/ this one might be better place – Andrey Jan 30 '11 at 20:50
  • Hmm ok, i was hesitating to put it there but decided not to in the end. Since i don't think i can *move* this, do i just delete it and open a new one there ? – Aerus Jan 30 '11 at 20:59
  • @Andrey: cstheory is for **research level** questions in **theoretical** computer science. "Research-level" means, roughly, questions that might be discussed between two professors, or between graduate students working on Ph.D.'s, but not usually between a professor and the typical undergraduate student. Please don't redirect programming questions to us, they are off-topic on cstheory. Thanks. – Kaveh Jan 31 '11 at 10:57
  • ps: **non-research-level** theoretical questions should be redirected to [Math.SE](http://math.stackexchange.com) and not to cstheory. – Kaveh Jan 31 '11 at 11:09
  • All you would need to do on the next line is introduce the identity a=a. The line after that is biconditional elimination using those two lines which gives you Cube(a). – Frank Hubeny Nov 24 '18 at 02:55

1 Answers1

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(I don't have a copy of Fitch and have never used it, so take this with a pinch of salt. But I'm pretty sure it's right.)

First get just "a=a" using =Intro. (You don't need any premises.) Then take that plus your 1. and apply <->Elim to get Cube(a).

Gareth McCaughan
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