21581971 has 4 divisors (see below), whose sum is σ = 21989232. Its totient is φ = 21174712.

The previous prime is 21581969. The next prime is 21582013. The reversal of 21581971 is 17918512.

It is a happy number.

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

It is a cyclic number.

It is not a de Polignac number, because 21581971 - 2¹ = 21581969 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21581951) 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, 203551 + ... + 203656.

It is an arithmetic number, because the mean of its divisors is an integer number (5497308).

Almost surely, 2^21581971 is an apocalyptic number.

21581971 is a deficient number, since it is larger than the sum of its proper divisors (407261).

21581971 is an equidigital number, since it uses as much as digits as its factorization.

21581971 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 407260.

The product of its digits is 5040, while the sum is 34.

The square root of 21581971 is about 4645.6399989668. The cubic root of 21581971 is about 278.4178239995.

The spelling of 21581971 in words is "twenty-one million, five hundred eighty-one thousand, nine hundred seventy-one".