253 (number) explained

253 (two hundred [and] fifty-three) is the natural number following 252 and preceding 254.

In mathematics

253 is:

• a semiprime since it is the product of 2 primes.
• the 22nd triangular number.^[1]
• a star number.^[2]
• a centered heptagonal number.^[3]
• a centered nonagonal number.^[4]
• a Blum integer.^[5]
• a member of the 13-aliquot tree.

is a prime number. Its decimal expansion is 252 nines, an eight, and 253 more nines.

Notes and References

1. triangular numbers.
2. Centered 12-gonal numbers. Also star numbers: 6*n*(n-1) + 1.
3. Centered heptagonal numbers. a(n) = (7*n^2 - 7*n + 2)/2.
4. Centered 9-gonal numbers. (3n-1)*(3n-2)/2.
5. Blum integers: numbers of the form p * q where p and q are distinct primes congruent to 3 (mod 4).