Asaf Karagila
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630/400 score
 15h reviewed Reopen Can a line be constructed from points? 17h comment Express the statement that $x$ has at least one element in the language of set theory I don't know what you were taught, $\exists !$ is a shorthand, and you may or may not be allowed to use it. But yes, other than that it's fine. 17h comment Express the statement that $x$ has at least one element in the language of set theory No, $\varnothing$ is not part of the language of set theory. The language of set theory has only $\in$ (and $=$ in modern treatments). How do you say that something has exactly one element? Now say that something has at most one element if it is empty, or has exactly one element. 17h revised Express the statement that $x$ has at least one element in the language of set theory edited tags; edited title; edited tags 17h answered Express the statement that $x$ has at least one element in the language of set theory 17h reviewed Reviewed return coefficients matlab 17h comment return coefficients matlab This seems to be more of a programming oriented question, and it might be better suited for StackOverflow instead. 17h reviewed No Action Needed Third Order Differential Equations 17h reviewed No Action Needed “Opposite” point on ellipsis by axis (or vector) 19h comment Is this an accurate layman's description of the Anti Foundation Axiom @Stefan: Yes. There are several variants of it (see en.wikipedia.org/wiki/Non-well-founded_set_theory), too. AFA, usually refers to Aczel's axiom, which asserts that for every directed graph (or something like that) there exists a unique realization of that tree. 20h answered Connected subsets of $\mathbb{R}$. 20h comment Applications of the Axiom of Regularity to non-set-theoretical Mathematics Yes, because I wrote that you essentially don't need it for "non-set-theoretical mathematics". 20h comment Applications of the Axiom of Regularity to non-set-theoretical Mathematics math.stackexchange.com/questions/213639/… 22h revised $\bigcup X$ finite implies $\mathcal P(X)$ is finite. added 4 characters in body; edited tags; edited title 22h answered $\bigcup X$ finite implies $\mathcal P(X)$ is finite. 22h revised What is the number of levels in Qubrix Brain Twister? edited tags 1d comment Definition of ordered ring flawed? Perhaps a slightly better title is in order. Perhaps something which doesn't come off as "mathematics is wrong and I'm right" and more along the lines of "I probably missed something, but I'm not sure what". 1d answered Definition of ordered ring flawed? 1d comment Every subset of a finite set is finite.. confused why this proof is wrong.. As @Clayton says. If you haven't proved that every subset of $[n]$ is finite, then you're using the statement that you need to prove. Therefore, you should first prove this for $[n]$ by showing that if $A\subseteq[n]$, then for some $m\leq n$, $A$ has a bijection with $[m]$. Then your proof is fine. 1d reviewed Reviewed How can you find two solutions to $\cos(3x - \frac{\pi}{2}) = 0$ by illustrating the situation on the unit circle?