21561031 has 4 divisors (see below), whose sum is σ = 21640864. Its totient is φ = 21481200.

The previous prime is 21561013. The next prime is 21561041. The reversal of 21561031 is 13016512.

21561031 = 136² + 137² + ... + 406².

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21561031 is a prime.

It is a super-2 number, since 2×21561031² = 929756115565922, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21561011) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 39510 + ... + 40051.

It is an arithmetic number, because the mean of its divisors is an integer number (5410216).

Almost surely, 2^21561031 is an apocalyptic number.

21561031 is a deficient number, since it is larger than the sum of its proper divisors (79833).

21561031 is an equidigital number, since it uses as much as digits as its factorization.

21561031 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 79832.

The product of its (nonzero) digits is 180, while the sum is 19.

The square root of 21561031 is about 4643.3857259547. The cubic root of 21561031 is about 278.3277494985.

Adding to 21561031 its reverse (13016512), we get a palindrome (34577543).

The spelling of 21561031 in words is "twenty-one million, five hundred sixty-one thousand, thirty-one".