21561506 has 4 divisors (see below), whose sum is σ = 32342262. Its totient is φ = 10780752.

The previous prime is 21561503. The next prime is 21561571. The reversal of 21561506 is 60516512.

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

It can be written as a sum of positive squares in only one way, i.e., 17297281 + 4264225 = 4159^2 + 2065^2 .

It is not an unprimeable number, because it can be changed into a prime (21561503) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 5390375 + ... + 5390378.

Almost surely, 2^21561506 is an apocalyptic number.

21561506 is a deficient number, since it is larger than the sum of its proper divisors (10780756).

21561506 is a wasteful number, since it uses less digits than its factorization.

21561506 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 10780755.

The product of its (nonzero) digits is 1800, while the sum is 26.

The square root of 21561506 is about 4643.4368736960. The cubic root of 21561506 is about 278.3297933821.

The spelling of 21561506 in words is "twenty-one million, five hundred sixty-one thousand, five hundred six".