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Nov
20
comment Probability of picking a certain option at least once in two tries
Your calculation gives the probability that he teleported to work and teleported back. That's what multiplying the probability of two events does.
Nov
14
comment If A is infinite, does there have to exist a subset of A that is equivalent to A?
Thanks for the clarifications, everyone. My curiosity is satisfied by the role of the AoC in the "obvious" construction. So Rudin's "is not finite" means "there is no bijection to an element of $\mathbb N$?
Nov
12
comment If A is infinite, does there have to exist a subset of A that is equivalent to A?
Is the axiom of choice really needed for the first step? Why?
Oct
23
comment Non-Euclidean Geometry for Children
Sounds like you can just give Flatland to your son, and he will make the effort of bringing it to bear.
Sep
10
comment What is exactly the difference between a definition and an axiom?
@Bye, the defining properties of a vector space don't need to hold axiomatically: For example, $\mathbb R \times \mathbb R$ is a vector space because of the predefined properties of addition and multiplication. You can also study vector spaces in the abstract (i.e. vector spaces have all the defining properties because they are vector spaces), but that's a different thing.
Sep
9
revised Finding x in an Olympiad simultaneous equation
deleted 7 characters in body
Sep
8
revised Finding x in an Olympiad simultaneous equation
added 185 characters in body
Sep
8
comment How to solve 3 variable in 2 equation?
"Arbitrary" does not mean "I get to choose a value"; it means "I don't choose a value, and I don't try to find one." Every value of $x$ leads to values for $y$ and $z$, and that's the complete solution unless there are additional constraints (e.g., if $x, y, z$ must be integers).
Sep
8
awarded  Nice Answer
Sep
7
awarded  Yearling
Sep
7
revised Finding x in an Olympiad simultaneous equation
added 1 character in body
Sep
7
answered Finding x in an Olympiad simultaneous equation
Jun
17
comment Is it a composite number?
Thanks, just upvoted Hagen's answer.
Jun
17
comment Is it a composite number?
Impressive, but can you indicate how you came up with these divisors, and with (mod 4) for the cases?
Jun
9
comment Why is zero the only infinitesimal real number?
@samuel, you should now choose the answer that helped you the most and "accept" it by clicking on the big check mark.
Jun
3
comment Name of the highest power of 2 smaller than or equal to a given number
It would in a programming context, but in mathematics we used to just talk about digits regardless of base. E.g., Cantor's diagonalization proof is about binary digits, not bits.
Jun
3
answered Name of the highest power of 2 smaller than or equal to a given number
May
20
awarded  Scholar
May
20
comment Restricting a relation to an injective function
That's great, thanks! Since it's a graph theory problem it figures that it would be intactable without the big guns... :-(
May
20
accepted Restricting a relation to an injective function