 Numberplay  The New York Times
Mathematical, logical and scientific puzzles, with solution "recaps." The posts, by Gary Antonick and Pradeep Mutalik, have included "Surreal Hat Tricks," "The Puzzles of Pickover," "Prime Rib at Hogwarts," "Nim for (a) Change," "Phi  The Magic and the
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 Number Puzzles  Erich's Puzzle Palace; Erich Friedman
Three crosswordgrid style puzzles, with numerical operations filled in. Can you fill in the numbers that work? Answers provided.
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 Number Puzzles  Ken and Dima Boone
Arithmetic puzzles along with hints and answers include "Laying Eggs," "Four Fours Into Seven," "Target Number 278," "Inserting Plus Signs," and "How Many Legs?" and "What's My Number?"
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 Numbers in the World (Math Chat)  Frank Morgan, MAA Online
Select the best occurrence in the world of each number from 1 to 10. For example, 12 is the number of eggs in a dozen or the number of months in a year... with winning reponses. Also: Math Chat has received a number of mathematical jokes, most of them
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 Numbers Test  Tommie Berry
A Java program for teaching children their math facts. Select the digits and functions (addition, subtraction, multiplication, and/or division) you want to practice, and hit the start button. The applet will generate questions for you, will tell you when
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 Number Strings  Imm, Kara; Lambert, Rachel; et al.
A place to find new strings, post strings, get feedback on design or number choice, and discuss the pedagogy of number strings, a classroom routine based on "sets of related math problems, crafted to support students to construct big ideas about mathematics
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 The Number Years  Libicki, Quinn
A concept originally created by Eric Libicki as a classroom activity to bring a sense of fun to learning the history of mathematics, now in the game show format presented here. Several online versions are available, and users will be able to craft their
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 Numerix Bee  Vel Micro Works Incorporated
A math challenge game designed to develop children's interest, confidence, and fluency in mathematics through solving challenging problems while having fun. Register your school.
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 Nummòlt  Maurici Carbó Jordi
Math teaching resources built by architects include base 10 block manipulatives; a block construction toybox; a logic construction toybox; an area workshop; and MathCats balance. See, in particular, Nummòlt's openended obbl & math toys: adding
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 The Official Scoring Site of the International Mathematical Olympiad 2001  Wolfram Research
The IMO 2001 competition, for students under the age of 20 who have had no postsecondaryschool education, was held July 114, 2001, in Washington, DC, USA. Each country represented is allowed up to six participants. This site records scores and supplies
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 Off The Hypotenuse  Glenn Kenyon
Kenyon started this blog in pursuit of "a good discussion of resources, ideas, and innovative ideas for teachers of Algebra." Humorous videos, comic strips, and posts, which date back to March of 2007, have included "Math Lessons learned from Karate,"
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 Ohio Proficiency and Graduation Practice Tests  Murtaza Ozdemir, Horizon Science Academy Online
Practice math questions culled from the last four Ohio sixth and ninthgrade proficiency tests or the Ohio graduation test. Users receive a detailed report of their score and a list of missed or skipped questions at the end of the test.
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 Old and New Arithmetic  Ivars Peterson (MathLand)
"Three merchants have invested their money in a partnership..." This problem appears in a mathematics textbook known as the Treviso Arithmetic. The original book, written in a Venetian dialect, had no formal title, and its author is unknown. Treviso is
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 Olimpíada Brasileira de Matemática  Sociedade Brasileira de Matemática (SBM)
The Olimpíada Brasileira de Matemática (OBM) is a competition open to all students enrolled in Brazil's fundamental or middle studies. Browse or download archived exam problems and solutions dating back to 1997. Read news about olympiads, or the journal
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 Olimpíadas Portuguesas de Matemática  Soc. Portuguesa de Matemática (SPM)
The Portuguese mathematical Olympiads: history; archive of problems; past winners; rules; the Portuguese team; links to relevant Web sites; and more.
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 Olimpiada Matemática Española  C. SánchezRubio
Convocatoria XXXV Olimpiada (Curso 1998/99); Comisión de la Olimpiada Matemática Española; Información para alumnos y profesores; Problemas de las últimas Olimpiadas (Fase nacional); Material para preparación;
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 Olimpiada Mexicana de Matemáticas  Luis Miguel García Velázquez
Mexican Math Olympiad Official Site. Mathematical Contest with problems,
tests, and other material. In Spanish. Información General de la Olimpiada. Problemas, exámenes y material de estudios. Contactos, noticias y mucho más. Also
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 Oliver Knill
Research, teaching, and media by this preceptor in Harvard's math department. Knill began in dynamical systems, tackling first ergodic and spectral theoretical questions, then probability theory and elementary number theory. This led to his "passion for
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 On Beyond Newton: from Simple Rules to Stability, Fluctuation, and Chaos (Serendip: Complex Systems)  Bryn Mawr College
An interactive exhibit on homeostasis/autonomy/chaos centered around population dynamics. The phenomenon of rules giving rise to not only order but also disorder, and, beyond that, to previously unknown forms of order, is the terrain currently being explored
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 Once in a Millennium (Math Chat)  Frank Morgan, MAA Online
Challenge: The year 2000 can be represented as the sum of consecutive integers:
2000 = 398 + 399 + 400 + 401 + 402. The year 2001 can be represented as the sum of consecutive integers: 2001 = 1000 + 1001. In fact, every year in the millennium from 2000
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 OnePage Proof of Fermat: Critique (Math Chat)  Frank Morgan, MAA Online
Critique the following short proof of Fermat's Last Theorem sent in by
reader Rob Connelly. (In perhaps the biggest mathematics news of the century, Andrew Wiles recently came up with a very long and complicated proof of this 350year old problem.) Answer:
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 One Trip to the Attic? (Math Chat)  Frank Morgan, MAA Online
In the lobby of a hotel there are three on/off switches. One is attached to a light bulb on the third floor; the other two are attached to nothing at all. You may work the three switches as often as you please. How many times do you need to visit the
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 On Four 4s and Speed Limits (Math Chat)  Frank Morgan; Christian Science Monitor
Can you get past 30 in forming numbers 0, 1, 2, 3, ... from four 4s and the standard mathematical operations +,  , x, /, decimal point, square root, powers, and factorial(!)?
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 OnLine Encyclopedia of Integer Sequences  The OEIS Foundation Inc.
Before publishing that new sequence you have just found, you might want to check here to see if it's already been discovered. Directions for how to look up a sequence in the table, abbreviations used, sending in a new sequence; how to look up a reference
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 Online games and activities at Syvum  Syvum Technologies, Inc.
Online activities for practicing practice addition, subtraction, multiplication, division, fractions, algebra, and word problems. Workbooktype activities are interactive, with automatic generation of tests, immediate grading, and a new set of problems
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 Online Ideas Maths Resources  James Bancroft
Problems in Microsoft Word, HTML, and JPG, organized into general resources; addition and subtraction; multiplication and division; numbers and the number system; fractions, decimals, percentages, ratio, and proportion; real life problems; measure and
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 Onlineindiaeducation  Atul Mittal
Page after page of questions with links to view answers and explanations: aptitude (area, series, "problems on trains," boats and streams, odd man out, and more), puzzles (sudoku, number, logical, clock, etc.), and reasoning (cause and effect, arithmetic,
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 online intervention.com  Pearson Education
onlineintervention.com assesses students' performance in specific skills and prescribes customized intervention activities, automatically generating summary reports for both teachers and students. Register to take online quizzes, play games, and use learning
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 Online Math Applications  Leung, Jiang, Jiang, Chiu, Klopfer, Chiu; ThinkQuest Junior
Activities, examples, problems, and online quizzes about math applied to real world situations in investing (a stock market simulation game), music (classical MIDI files), history (biographies and anecdotes), science (astronomy, computers, 'math in the
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 Online Math Education  Yordan Petrov Petrov
Math tutorials and quizzes, with some math history information as well. Includes a discussion forum. Available in English or Bulgarian.
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 Online Math Help for All  Ramachandran Seerangan
Primers and problems, a math dictionary, worksheets organized by grade and concept, and more.
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 Online Math Lab  Eric Bitter, Clovis Unified School District  CAL Online
Modules on fractions, decimals, and percents offer tutorials and problems for students in grades 412.
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 Online Math Tests Home Page  Elias Saab
This site has problems that help sutudents get ready for College Alegbra or Calculus. It also has problems in Geometry and Trigonometry. All the problems have complete solutions that are given when one hits the submit button. Tests covered include: MPE410,
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 Online Math Worksheets  Improving Education Inc.
Random generated K5 math worksheets  each time you load a Web page, new exercises are generated. Match the numbers; 1, 2, and 3digit addition and subtraction; multiplication and division, including long division.
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 Online Resources  Georgia Tech School of Math
Syllabi, course notes, problems, and Maple worksheets for single and multivariable calculus, linear algebra, statistics, and a Math for Management course (statistics, probability, sets, and some calculus). The site also contains sample exams for calculus
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 The Online Test Page  Elias Saab
Math and vocabulary drills, including MathCOUNTS, SAT, ACT, and Missouri MMPT questions, as well as links to relevant sites.
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 On Motivation and Understanding  Cut the Knot!, Alexander Bogomolny
Fourth in a series of columns that draw on Frank Morley's fundamental paper On the Metric Geometry of the Plane nLine  dealing with what amounts to his proof of the Trisector Theorem. Bogomolny outlines Morley's reasoning first and then supplies necessary
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 On the 3x + 1 Problem  Eric Roosendaal
Numerical data and some conjectures on this problem, also known as the Collatz problem or the Syracuse problem. All numbers up to 29,300 * 10^12 (~ 26 * 2^50) have been checked for convergence.
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 On the Puzzles with polyhedra and numbers  Jorge Rezende
Several articles in PDF format, with related links: one on a set of didactic puzzles published by the Portuguese Mathematical Society (SPM); "Jogos com poliedros e permutações", an article in Portuguese describing puzzles with polyhedra
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 On Thin Spaghetti and Nocturnal Animals  Don Cohen (The Mathman)
Function and Graphs for Children. Guess My Rule; Graphing; The Shuttle Puzzle; The Tower Puzzle; The Nautilus Shell. Sample problems with some answers from Chapter 6 of Don Cohen's worksheet book.
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 On This Day in Math  Pat Ballew
Mathematically significant events, births, deaths, and more, posted every calendar day. Ballew's blog reveals his thoughts on teaching math. His links and explanations written using The Geometer's Sketchpad include triangle medians; angle bisectors in
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 OOG: the Object Orientation Game  MCM Productions
MCM Productions' first publicly released software application. This 4in1 tiling puzzle game for the MSWindows environment allows players to test their problem solving ability with over 300 puzzles, including Tangrams, Pentominoes, Hexagons and Polyominoes.
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 .oO worksheeps  Felix Heckert
Generate worksheets of customized math exercises, complete with solutions: basic operations with integers, negatives, and fractions; binomial and quadratic formulas; partial roots; percent calculation; curve sketching; metric; derivations; antiderivatives;
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 OpenEnded Math Problems  Gwenn Holtz and Mary Lee Malen, Levering School and the Franklin Institute
An eightmonth slate of openended problems for middle school students to solve in preparation for standardized testing. The authors have composed and selected problems that lend themselves to multiple solution paths, and then organized them into three
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 OpenIntro  David Diez and Chris Barr, CoFounders
The OpenIntro Statistics textbook the inaugural effort from these makers of educational products that "are free, transparent, and lower barriers to education"  comes with data sets for the statistics software package R; normal, t, and chisquare probability
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 Open Middle  Johnson, Nanette; and Kaplinsky, Robert
"Open middle" problems, hints, and answers, all aligned to the Common Core State Standards (CCSS). Introduced by Dan Meyer, these kinds of math challenges have a "closed beginning," meaning that they all start with the same initial problem; a "closed
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 Open Problem Garden  Matt DeVos and Robert Šámal
A collection of unsolved problems  mainly, graph theory, combinatorics, and number theory. A wiki for each open problem includes relevant bibliographical citations, importance rating, and recommendation for undergraduates. Navigate by math subject,
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 Open Problems for Undergraduates (DIMACS)  Robert Hochberg, Center for Discrete Mathematics and Theoretical Computer Science, Rutgers Univ.
A collection of open problems in Discrete Mathematics in the areas of Graph Theory; Combinatorial Geometry; Geometry/Number theory; and Venn Diagrams, currently being researched by members of the DIMACS community. These problems are easily stated, require
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 Open Problems in Dynamical Systems  Dept. of Mathematics, State Univ. of New York at Stony Brook
A collection of papers outlining unsolved problems in dynamical systems.
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 Open Problems in Dynamical Systems & Ergodic Theory  Sergiy Kolyada
This collection of problems originated in the Katsiveli2000 Open Problems Session (Chairman  Benjamin Weiss). New problems are being added. Copies of the page are also at http://iml.univmrs.fr:80/~kolyada/opds/ and http://www.math.iupui.edu/~mmisiure/open/.
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