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I want to formalize the following sentence in predicate logic:

If a children has spinach or mushrooms on its pasta then it will not eat its pasta.

The headline contains a shorter version. I have four solutions where l like your opinion or recommendation on:

\begin{align} \forall x \forall y \, &((IsChildren(x) \land IsPasta(y) \land {} \\ &\land (hasIngridient(y,Spinach) \lor hasIngridient(y,Mushroom)) \rightarrow \neg Eatspasta(x,y) ) \end{align}

  1. Modification: Could I omit $IsChildren(x)\land IsPasta(y)$ though naming $x$ in child and $y$ in pasta? For example: \begin{align} \forall child \forall pasta \, &((hasIngridient(pasta,Spinach) \lor hasIngridient(pasta,Mushroom)) \\ &\rightarrow \neg EatsPasta(child,pasta) ) \end{align}

Q1.1 Are variables allowed with more then one letter?

Q1.2 I think the name of a variable (is child or $x$) does not matter for its interpretation. So after the first modification child could be anything from my discourse universe.

  1. Modification: Could I omit $\forall y$ by changing the formalization as follows: \begin{align} \forall x \, &(isChilrden(x) \land (hasIngridientSpinachOnPasta(x) \lor hasIngridientMushroomOnPasta(x)) \\ &\rightarrow \neg EatsPasta(x) ) \end{align}

  2. Modification: Could I omit $\forall y$ by changing the formalization as follows: \begin{align} \forall x \, &(isChilrden(x) \land (hasIngridientSpinach(Pasta) \lor hasIngridientMushroomOnPasta(Pasta)) \\ &\rightarrow \neg EatsPasta(x,Pasta) ) \end{align}

  3. Modification: Could I say instead there exist no chilrden that eats pasta with Spinat und Mushrooms: $$\neg \exists x \forall y x(isChilrden(x) \land (Pasta(y,Spinach) \lor Pasta(y,Mushroom)) \rightarrow EatsPasta(x,y) )$$

Main question: which one are incorrect and why?

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  • $\begingroup$ On reading the title I had to double-check that I hadn't landed on parenting.stackexchange.com by mistake :) $\endgroup$ – Rahul Jul 25 '18 at 9:38
  • $\begingroup$ @Rahul unfourtenatly it does not help to bring up more answers. So I changed it back... $\endgroup$ – user3352632 Jul 26 '18 at 13:15
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Q1.1 Are variables allowed with more then one letter?

It's unusual in maths; usually a one-letter symbol with descriptive index is preferred, such as $n_\mathrm{mushrooms}$. But IMO multi-character names are fine, as are multi-character functions ($\sin$, $\log$...). But LaTeX won't typeset these properly if you just write out the letters like that; notice the bad kerning in your post. You should instead wrap such names in a suitable \math??{.} styling. Options are:

  • $Spinach$ ($Spinach$) (bad!)
  • $\mathit{Spinach}$ ($\mathit{Spinach}$)
  • $\mathrm{Spinach}$ ($\mathrm{Spinach}$)
  • $\mathbf{Spinach}$ ($\mathbf{Spinach}$)
  • $\mathsf{Spinach}$ ($\mathsf{Spinach}$)
  • $\mathtt{Spinach}$ ($\mathtt{Spinach}$)

I would recommend \mathit for multi-character variables, \mathrm for constants and standard functions, and \mathsf or \mathtt for special situations (like when you're paraphrasing computer code).

Q1.2 I think the name of a variable (is child or x) does not matter for its interpretation

Correct. This is what's in CS called $\alpha$-conversion: any variable that's quantified over with $\forall$ or $\exists$ can be renamed.

Your modifications 2 and 3 don't quite express the original thing, because you don't quantify over the pasta anymore. Basically this means that the pasta is a “global constant”, you only ever consider one. This is often done in maths/science for simplification, but logic-wise it does make a difference.

Modification 4 just turns around the quantors. This can always be done in classical logic: $$ (\forall x. \neg P(x)) \leftrightarrow \neg(\exists x. P(x)) $$

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  • $\begingroup$ Thanks for your comprehensive reply. Regarding Modification 2 and 3: do you think in consideration of the original sentence that it is necessary to quantify over pasta? $\endgroup$ – user3352632 Jul 26 '18 at 13:14

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