- Two identical, similar, or corresponding things that are matched for use together:a pair of gloves; a pair of earrings.
- A word or phrase that reads the same backwards as forwards, for example, madam, nurses run, Madam, I’m Adam or Poor Dan is in a droop.
palindromic numbers 回文数
- The number likes 1347431, 20058385002.
- A plane curve formed by the intersection of a right circular cone and a plane parallel to an element of the cone or by the locus of points equidistant from a fixed line and a fixed point not on the line. It like the path of an object thrown into the air and falling back to earth.
- Two or more straight coplanar lines, planes, or a line and a plane that do not intersect. Or designating curves or surfaces everywhere equidistant.
- A quadrilateral having both pairs of opposite sides parallel and equal to each other.
- A constant or variable term in a function that determines the specific form of the function but not its general nature, as a in f(x) = ax, where a determines only the slope of the line described by f(x).
- One of the independent variables in a set of parametric equations.
- Statistics: a variable entering into the mathematical form of any distribution such that the possible values of the variable correspond to different distributions.
- Computers: a variable that must be given a specific value during the execution of a program or of a procedure within a program.
- One of a pair of shallow, curved signs, ( ), used to enclose an additional inserted word or comment and distinguish it from the sentence in which it is found.
- A decorative design, as for wallpaper, china, or textile fabrics, composed of elements in a regular arrangement.
- A polygon with five sides and five angles. wikipedia-Pentagon
pentagonal number 五角数
- A pentagonal number is a figurate number that extends the concept of triangular and square numbers to the pentagon, but, unlike the first two, the patterns involved in the construction of pentagonal numbers are not rotationally symmetrical. The nth pentagonal number pn is the number of distinct dots in a pattern of dots consisting of the outlines of regular pentagons with sides up to n dots, when the pentagons are overlaid so that they share one vertex. For instance, the third one is formed from outlines comprising 1, 5 and 10 dots, but the 1, and 3 of the 5, coincide with 3 of the 10 – leaving 12 distinct dots, 10 in the form of a pentagon, and 2 inside. 1, 5, 12, 22, 35, 51, 70, 92, 117, 145, 176, 210, 247, 287, 330 …
- A five-pointed, star-shaped figure made by extending the sides of a regular pentagon until they meet, used as an occult symbol by the Pythagoreans and later philosophers, by magicians, etc.
- A solid figure having five faces.