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Help would be appreciated. The notes are poor on the subject, and I am clueless.

Verify the following equalities:

  • Verify the equality $$\mathsf{SIII}=_β\mathsf{I}$$ where $\mathsf{S} = λxyz.(xz)(yz) \quad \mathsf{I}= λx.x$

  • Verify the equality $$\operatorname{twice} (\operatorname{twice}) f x =_β f(f(f(f x)))$$ where $\operatorname{twice} = λfx.f(f x)$

2 Answers 2

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Try substituting the arguments in the definitions of the combinators.

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    When I originally tried that I got (x x)(x x) = λx.x and λfx.f(f x)(λfx.f(fx))fx=f(f(f(fx)))2010-11-23
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    SIII=(II)(II)=II=I2010-11-23
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    twice twice f x = twice (twice f) x = twice = twice f (twice f x) = twice f f(f(x) = f(f(f(f(x))))2010-11-23
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    You just have to keep substituting.2010-11-23
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    I'm sorry (especially for the necropost), I don't understand your solution to b. you said... twice twice f x = twice (twice f) x -- how is that true? don't you have to apply the first twice before you can link the f to the second twice? After the first step, I find myself with: lambda(x) (twice(twice(x))2012-01-21
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    @Daniel What happened to your $f$?2012-01-22
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    Note that application associates to the left, so twice twice f x really means (((twice twice) f) x)2012-01-22
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I am posting this answer to make solutions more clear. $$ \mathsf{SIII}=_\beta (λxyz.(xz)(yz)) \mathsf{III} =_\beta (\mathsf{II})(\mathsf{II}) $$ Remember that $\mathsf{I}=\lambda x.x$ is the operator that does nothing. This means that for every lambda expression $M$, you have $\mathsf{I}M =_\beta M$.

This implies that: $$\mathsf{II} =_\beta (\lambda x.x)(\lambda x.x)=\lambda x.x = \mathsf{I}$$ And so we have $$ (\mathsf{II})(\mathsf{II}) =_\beta (\mathsf{I})(\mathsf{I}) =_\beta = \mathsf{II} =_\beta \mathsf{I}$$

The second one is easier

$$ \operatorname{twice} \operatorname{twice} f x = \operatorname{twice} (\operatorname{twice} f) x = \operatorname{twice} f (\operatorname{twice} f x) = \operatorname{twice} f f(f(x) = f(f(f(f(x)))) $$