Math Reference Project
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Welcome to the Math Reference project, an electronic archive of mathematical information.  Topics range from high school geometry up to graduate level topology, and (almost) everything in between.  You can either search by topic, scan through the site map, or search by keywords.  Click here for our privacy policy.

All material in these pages is protected, © Karl Dahlke, 2002-2006, and may not be copied or redistributed without written consent.  You may contact us at any time with questions or corrections, or join our mailing list.  If you're a new visitor, you can skip navigation, and read more about the Math Reference project below.

Number Theory    Plane Geometry    Combinatorics    Graph Theory    Limits, Continuity, Sequences, Series    Linear Algebra, Vector Spaces, Matrices    Calculus    Complex Variables and Analytic Functions    Logic, Proofs, Consistency, and Completeness    Set Theory    Point-Set Topology    Algebraic Topology    Group Theory    Ring Theory    Modules    Commutative Rings and Integral Domains    Field Theory    Algebraic Number Theory    Algebraic Geometry    Probability, Density and Distribution Functions    Elliptic Curves    Languages, Grammars, Machines, Complexity Theory    Category Theory

Introduction

This website assumes you know the basics, i.e. high school algebra and geometry.  If you're patient, and you have some innate talents, a solid high school background is enough to begin most topics.  However, you probably can't descend into any of these topics in detail until you've covered most of them at a high level.  If you're just starting out, you sort of have to learn math breadth first, rather than depth first, since areas of math tend to call upon one another as they get more advanced.

For now, all information is in html.  This means I don't use some of the strange symbols commonly employed in higher mathematics.  Fortunately, we won't need those for a while, and in the meantime this site remains accessible to all.  Even a blind user should be able to access this site using edbrowse.

Although I am avoiding, or at least postponing, some of the more complex notation, we will still need some basic symbols such as × and ÷, and the Greek alphabet.  I am using the unicode standard to represent these symbols - I hope your browser is compatible.

I try to use the times symbol × for arithmetic multiplication, and * for generic multiplication in a group or ring.  On rare occasions, × denotes ordinal or cardinal multiplication, a natural extension of integer multiplication.  When applied to modules, u×v is the tensor product of u and v.  When * is applied to sets it indicates their cross product, or their direct product if you prefer.

The symbol "/" and the word "over" both indicate division, but over is a lower precedence operator, and is usually used for fractions.  Thus (a+b)/(c+d) is the same as a+b over c+d.  I try to put spaces around the lower precedence operators such as = and over, to make the equations a little clearer.

Of course / means many more things, including quotient group, quotient ring, quotient module, fraction ring, quotient space, field extension, integral extension, and so on.  It is the most overloaded of all the operators.  Sorry about that.

If f and g are functions, I will sometimes write fg for f followed by g.  Of course, if parentheses are used, they rule the day.  Thus fg applied to x is g(f(x)).  In other contexts, fg might mean g followed by f, but this is not the typical interpretation of juxtaposition (on my website), so I'll try to be very explicit when fg means g compose f.

When I say "assume" x = 3, then x must equal 3 for this theorem, or this part of the argument, to hold.  If I say "suppose" x = 3, this is a proof by contradiction, and I will soon show that x cannnot equal 3.

Variables that stand for numbers, or points, or functions, are in lower case.  Variables that represent sets are in upper case.  So you might see: let x be an element of the group G.  I won't let g be an element of the group G, as that is simply too confusing.  However, I may let g₁ and g₂ be two elements of G, whence g₁*g₂ is also in G.

Bold text is reserved for fixed mathematical structures, like the reals R, or the rationals Q, or the alternating group on 9 letters A₉, or the n sphere Sⁿ.  Thus the loops in a topological space S are the continuous maps from S¹ into S.

You'll notice that each page in this archive begins with a light blue navigation panel.  The down arrows take you to subtopics, areas of math that are "spin-offs" or specializations of the current topic.  The up arrows take you back to higher level topics.  There are no up arrows here, since this is the main page.  You might follow down arrow links to "calculus", then to "arc length and curvature", then take the up arrows back to calculus, then back to this page.  If you are used to directory structures (Unix), or folders within folders (Windows), this layout should seem natural.  A topic is like a folder, and the web pages within that topic are the files in the folder.  They explain the topic in detail, and should be read sequentially, starting with the introduction.  you can step forward and backward through the pages of a given topic using the left and right arrows at the bottom of each page.  Thus you need not retrieve the entire archive on integral calculus just to review integration by parts.  This saves time for the folks with dial-up connections, and if you're doing a keyword search, you see only the information you requested.  However, it that information is unclear when taken out of context, you may need to back up a couple of pages, or retreat to the introductory page and start from there.  The navigation panel always provides a link to the start of the current topic, i.e. the introductory page.

Topics below the current topic are only accessible from the introductory page.  If you're reading through that topic, well, I figure you don't want to descend, so why clutter the pages with additional links?  You can always jump back to the intro page, then descend to a lower level.  Two clicks instead of one; that's not bad.

The pages within a topic are also called sections.  This is less ambiguous, for a web page, when printed out, is often several pages long.  So I may refer you to "the previous section", or "an earlier section" in the current topic, or perhaps "another section" in a different topic.  These references usually include direct links, so you don't have to scroll down and use the arrows.

When you use the search function, don't bother with punctuation; I throw it all out.  There is no "advanced search".  Try to use words, rather than ascii equations.  Ask for "pythagorean triples", rather than "integers x^2 + y^2 = z^2".

This website requires javascript, but it does not employ cookies or plugins.  The javascript is minimal, and is supported by virtually every browser.

Advertising appears in yellow, like the box at the top of this page, so you can easily skip past it if you wish.  (I'm not pushy; no popups, no popunders, and no blinking animations to distract you.)  But remember, I have to make a living too, so please click on these links if they interest you.  Thank you.