Recreational mathematics

two-dimensional surface with only one side and only one edge

sequence of digits in the math constant π, incorrectly attributed to Feynman

mathematics carried out for recreation rather than as a strictly research and application-based professional activity

optical illusion used in mathematics

finite sum of distinct unit fractions

certain type of mistaken proof

visible regularity of form found in the natural world

mathematical problem about moving the largest possible shape around a 90° corner

thumb|A sangaku dedicated to Konnoh Hachimangu (Shibuya, Tokyo) in 1859.

mathematical problem of tiling an integral square using only other integral squares

type of puzzle

mathematical problem

FRACTRAN is a Turing-complete esoteric programming language invented by the mathematician John Conway. A FRACTRAN program is an ordered list of positive fractions together with an initial positive integer input n. The program is run by updating the integer n as follows: for the first fraction f in the list for which nf is an integer, replace n by nf repeat this rule until no fraction in the list produces an integer when multiplied by n, then halt.

mathematical problem

exponential function

a coincidence in mathematics

problem in number theory

problem in geometry

programming language or computer interface that allows for flexibility in function but is difficult to learn and use because it offers little or no support for common tasks

mathematical problem

mathematical problem

simplified example problem used for research or exposition

1938 novel by Júlio César de Mello e Souza

any number that is not an integer but is very close to one

the computational problem of choosing as few coins as possible that add up to a given amount of money

riddle

journal

mathematics problem

thumb|Example grid for a cross-figure puzzle with some answers filled in A cross-figure (also variously called cross number puzzle or figure logic) is a puzzle similar to a crossword in structure, but with entries that consist of numbers rather than words, where individual digits are entered in the blank cells. Clues may be mathematical ("the seventh prime number"), use general knowledge ("date of the Battle of Hastings") or refer to other clues ("9 down minus 3 across").

Unsolved geometry problem about planar regions

thumb|A sample puzzle Str8ts is a logic-based number-placement puzzle, invented by Jeff Widderich in 2008. It is distinct from, but shares some properties and rules with, Sudoku. The name is derived from the poker straight. The puzzle is published in a number of newspapers internationally, in two book collections, and in downloadable apps. It was featured on the Canadian television show ''Dragons' Den'' on November 24, 2010.