180 (number) two square numbers: 12 + 6. It can be expressed as either the sum of six consecutive prime numbers: 19 + 23 + 29 + 31 + 37 + 41, or the sum of eight consecutive prime numbers: 11 + 13 + 17 + 19 + 23 + 29 + 31 + 37. 180 is an Ulam number, which can be expressed as a sum of earlier terms in the Ulam sequence only as 177 + 3. 180 is a 61-gonal number. Half a circle has 180 degrees. Summing Euler's totient function φ("x") over the first + 24 integers gives
Weakly prime number Weakly prime number In number theory, a prime number is called weakly prime if it becomes not prime when any one of its digits is changed to every single other digit. Decimal digits are usually assumed. The first weakly prime numbers are: For the first of these, each of the 54 numbers 094001, 194001, 394001, ..., 294009 are composite. A weakly prime base-"b" number with "n" digits must produce ("b"−1) × "n" composite numbers when a digit is changed. In 2007 Jens Kruse Andersen found the 1000-digit weakly prime (17−17)/99 + 21686652. This is the largest known weakly prime number