21573509 has 2 divisors, whose sum is σ = 21573510. Its totient is φ = 21573508.

The previous prime is 21573469. The next prime is 21573529. The reversal of 21573509 is 90537512.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 12271009 + 9302500 = 3503^2 + 3050^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21573509 is a prime.

It is a super-3 number, since 3×21573509³ (a number of 23 digits) contains 333 as substring.

It is a Sophie Germain prime.

It is a Chen prime.

It is a Curzon number.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21573529) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10786754 + 10786755.

It is an arithmetic number, because the mean of its divisors is an integer number (10786755).

Almost surely, 2^21573509 is an apocalyptic number.

It is an amenable number.

21573509 is a deficient number, since it is larger than the sum of its proper divisors (1).

21573509 is an equidigital number, since it uses as much as digits as its factorization.

21573509 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 9450, while the sum is 32.

The square root of 21573509 is about 4644.7291632559. The cubic root of 21573509 is about 278.3814312837.

The spelling of 21573509 in words is "twenty-one million, five hundred seventy-three thousand, five hundred nine".