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Browse College Logic and Set Theory

Stars indicate particularly interesting answers or good places to begin browsing.

Infinite Sets [09/24/1997]
In my algebra class we have been debating whether the integers or the whole numbers contain more elements...

Integer Logic Puzzle [04/22/2001]
Two integers, m and n, each between 2 and 100 inclusive, have been chosen. The product is given to mathematician X and the sum to mathematician Y... find the integers.

Inverses within Semigroups [05/06/2002]
I would like to know the value of e in a semigroup for exponentiation, if it exists. In other words what CONSTANT value e satisfies the equation, a^b = e?

Is There a Universal Set of All Numbers? [06/16/2004]
The real numbers and the imaginary numbers are subsets of the complex numbers. Is the set of complex numbers a subset of a more universal set? Is there a universal set of all numbers agreed upon today?

Karnaugh Maps [05/07/2000]
What are Karnaugh maps? How are they used?

Lewis Carroll's Logic Problems [01/15/1997]
Where can I find out more about Lewis Carroll's logic problems?

Linear Topology [02/09/2003]
If a point in set X is finite, then X has a first point and a last point. Prove by induction if true, and give a counterexample if false.

Logical Sentences and Logical Arguments [05/16/2008]
What is the difference between A |- B and A -> B? They seem to mean the same thing--if A is true then you know that B is also true.

Logic and Conditional Sentences [10/04/2005]
I am having a hard time understanding why two false statements in a conditional sentence makes it true.

Logic: Bayes and Popper [06/24/2003]
Is p -> q totally equivalent to ~q -> ~p in practice?

Mathematical Logic [02/09/2001]
Assumptions, rules, contradictions, and a derivation.

Math Symbols [04/07/1997]
What do the common math symbols (backward E, upside-down A, etc.) mean?

The Meaning of 'Or' in Logic Statements [12/19/2003]
If a logic statement says, 'James is taking fencing or algebra,' does that mean he is taking one class or the other, or could he be taking both of them?

Necessary and/or Sufficient [05/26/2002]
What does it mean to say that a condition is necessary, sufficient, or necessary and sufficient?

Necessary and/or Sufficient Conditions with Modular Math [12/01/2006]
I'm working on a question in modular math that asks me to identify whether given conditions are "necessary", "sufficient", or "necessary and sufficient". I'm not sure what those terms mean.

One-to-One Correspondence of Infinite Sets [03/26/2001]
How can I prove that any two infinite subsets of the natural numbers can be put in a 1-1 correspondence?

Order of Quantifiers [12/19/2002]
Can you help me understand the order of quantifiers?

Orders of Infinity [12/05/2001]
I recently read a book about infinity which set forth several arguments for why there are different sizes or orders of infinity. None of them seem convincing to me...

Ordinals: Sets or Numbers? [04/16/2010]
Are ordinals sets or numbers? Doctor Tom resolves confusion around how to think about ordinals by putting them in the context of Zermelo-Fraenkel (ZF) set theory.

Problem from Real Analysis [10/05/2002]
Let X = A U B where A and B are subspaces of X. Let f:X->Y. Suppose that the restricted functions f|A:A->Y and f|B:B->Y are continuous. Show that if A and B are closed in X, then f is continuous.

Proof by Contradiction [04/29/2003]
Is there any specific mathematical theory that states that Proof by Contradiction is a valid proof?

Proof of One Step Subgroup Test [05/13/2002]
Prove that a nonempty subset H of a group G is a subgroup of G if and only if a*b^(-1) is in H for all a, b in H.

Proof Styles--Contradiction and Direct [04/22/2006]
Under what circumstances is it easier to prove a mathematical problem by the method of contradiction versus direct proof? I know the contradiction proof that sqrt(2) is irrational. Can you prove that by direct deduction?

Proof that 1 + 1 = 2 Using Peano's Postulates [09/12/2002]
How do I prove that 1 + 1 = 2?

Proof that f(K) is a Subgroup of G' [11/26/2001]
If G and G' are groups, f is an isomorphism from G into G', and K is a subgroup of G, then show that the set f(K)={f(k)\k is a member of K} is a subgroup of G'.

Properties of Relation [05/28/2003]
What are reflexive, symmetric, anti-symmetric, and transitive relations? star, please

Proving a Topology [10/11/1998]
Let X be an uncountable set of points, and T consist of the empty set and all subsets of X whose complement is finite. Prove that T is a topology of X.

Russell Paradox [7/2/1996]
I'm looking for a demonstration of the Russell Paradox (there is no ensemble of all ensembles).

Sets N, R, C, Z, and Q [01/22/2001]
What are the exact and extensive definitions of the sets N, R, C, Z and Q? What relation do these sets bear to one another?

Significance of Rational Numbers [01/11/2003]
Why are rational numbers defined the way they are?

Solving the Equation x^y = y^x [12/09/2004]
Solve x^y = y^x for x in terms of y only. Also, how do I find all possible solutions beyond the obvious ones of x = y, (2,4), and (4,2)?

Subsets and Subspaces [09/07/2001]
I'm trying to extract some relations in set notation from a description from Loomis and Sternberg's _Advanced_Calculus_...

Sum of Uncountably Many Positive Numbers [09/15/2004]
Let S be a set of uncountably many positive numbers. I would like to show that the sum of all the elements in S is infinite. That is, there is no convergent sum of uncountably many positive terms.

Transfinite Numbers [11/07/1997]
I know that Georg Cantor discovered transfinite numbers, but what are they?

Truth of a Biconditional Statement [11/08/2005]
Let p represent x = 0, and let q represent x + x = x. Write the biconditional p <-> q in words. Decide whether the biconditional is true.

Truth of the Contrapositive [06/07/2003]
The inverse of a statement's converse is the statement's contrapositive. Why?

Understanding the Transitive, Reflexive, and Symmetric Properties [06/30/2008]
Decide if the relation 'is not equal to' is a)transitive, b)reflexive, and c)symmetric with regard to the counting numbers.

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