Strobogrammatic number Strobogrammatic number A strobogrammatic number is a number whose numeral is rotationally symmetric, so that it appears the same when rotated 180 degrees. In other words, the numeral looks the same right-side up and upside down (e.g., 69, 96, 1001). A strobogrammatic prime is a strobogrammatic number that is also a prime number, i.e., a number that is only divisible by one and itself (e.g., 11). It is a type of ambigram, words and numbers that retain their meaning when viewed from a different perspective, such as palindromes. When written using standard characters (ASCII), the numbers, 0, 1, 8 are
Infinity symbol notation for summations and limits such as the infinity sign is conventionally interpreted as meaning that the variable grows arbitrarily large (towards infinity) rather than actually taking an infinite value. The infinity symbol may also be used to represent a point at infinity, especially when there is only one such point under consideration. This usage includes, for instance, the infinite point of a projective line, and the point added to a topological space formula_2 to form its one-point compactification formula_3. In areas other than mathematics, the infinity symbol may take on other related meanings; for instance, it has been used