MJD
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 2d comment In classical logic ~~p -> p? Intuitionistic? $\sim(\sim p)\to p$ is classically valid but not intuitionistically valid. $p\to \sim(\sim p)$ is both classically and intuitionistically valid.. 2d comment What exactly is an “analytic function”? Yes, you have it exactly right. The point isn't that you would represent a non-analytic function with a power series; as you observed, that would be silly. The point is that some functions can't be so represented, and it's useful to have a special term to distinguish functions that can be so represented from functions that can't. Apr 24 comment Regularity check of languages Here are some examples: math.stackexchange.com/questions/871662/… Apr 24 comment Regularity check of languages I think you don't understand the $/$ operation. It is not concatenation and $L_1/L_2$ is not what you said. Apr 23 comment Intuition for $\Bbb S^3 = \Bbb R^3 \cup \{ \infty \}$ Earlier discussion of this: math.stackexchange.com/questions/329391/… Apr 19 comment Examples of bijective map from $\mathbb{R}^3\rightarrow \mathbb{R}$ You should post that as an answer. Apr 18 comment Unit cube cut into two parts through its diagonals Have you tried drawing a picture? Have you tried taking a cube (say, a die) and marking its outside with a marker to show where the cut goes? Have you tried making a cube out of fruit and cutting it in the required way? Apr 15 comment Want to check if my Boolean Algebra simplification is correct How to format mathematics on this web site Apr 13 comment “Anti-Gray codes” that maximize the number of bits that change at each step If OEIS publishes this, it will appear as A271771. Apr 10 comment Proof that $\partial(A\times B) = ((\partial A)\times B)\cup (A\times(\partial B))$ For an imtuition, consider the product of a closed disc and a closed interval. This is a cylinder. The boundary of the cylinder has three components. The top and bottom of the cylinder are the product of the disc with the endpoints of the interval. The curved main tube of the cylinder is the product of the whole interval with the circle that bounds the closed disc. Apr 3 comment What's the probability to win (or lose) this solitaire? Closely related: Solitaire combinatorics Apr 2 comment Can proof by contradiction 'fail'? Lots of things are known to be independent. For example, each of the five Peano axioms is independent of the other four. Independence is not exceptional. Mar 31 comment What is the closure of x = y in the river metric? I think the river metric has $d(\langle x_1, y_1\rangle, \langle x_2, y_2\rangle) = |y_1| + |x_2-x_1| + |y_2|$. (Go from the first point perpendicularly to the river, go along the river, then go to perpendicularly to the second point.) Mar 30 comment Errors in math research papers Also Mar 30 comment show ((p → q) ^ (r → s) ^ (p v r )) → (q v s) is a tautology De Morgan's rule is not the rule that says that $p\to q$ is equivalent to $\sim p \lor q$. It is the rule that says that $\sim(p\land q)$ is equivalent to $\sim p\lor\sim q$. You need to use De Morgan's rule. Mar 30 comment show ((p → q) ^ (r → s) ^ (p v r )) → (q v s) is a tautology It's better if you show what you did so that someone can point out the error. Otherwise, you're just asking us to do your homework for you. Mar 28 comment group theory for non-mathematicians Compared with most advanced mathematics, basic group theory is not very difficult. Your friend might be surprised at how much success she has with a regular undergraduate textbook. The John Fraleigh book is very readable. Mar 21 comment Why is the sin of n times pi always 0 I don't understand the downvotes. This is a great question. Mar 21 comment Number of vertices of a complete graph with $n$ edges Mar 21 comment Division by zero reasoning. This has been discussed here many, many times. For example, Why not to extend the set of natural numbers to make it closed under division by zero? has an extensive discussion of which field axioms you would have to abandon to allow division by zero. The answer: most of them.