21512509 has 2 divisors, whose sum is σ = 21512510. Its totient is φ = 21512508.

The previous prime is 21512507. The next prime is 21512527. The reversal of 21512509 is 90521512.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 21187609 + 324900 = 4603^2 + 570^2 .

It is a cyclic number.

It is not a de Polignac number, because 21512509 - 2¹ = 21512507 is a prime.

Together with 21512507, it forms a pair of twin primes.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21512501) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10756254 + 10756255.

It is an arithmetic number, because the mean of its divisors is an integer number (10756255).

Almost surely, 2^21512509 is an apocalyptic number.

It is an amenable number.

21512509 is a deficient number, since it is larger than the sum of its proper divisors (1).

21512509 is an equidigital number, since it uses as much as digits as its factorization.

21512509 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 900, while the sum is 25.

The square root of 21512509 is about 4638.1579317656. The cubic root of 21512509 is about 278.1188052149.

The spelling of 21512509 in words is "twenty-one million, five hundred twelve thousand, five hundred nine".