21213581 has 4 divisors (see below), whose sum is σ = 21504252. Its totient is φ = 20922912.

The previous prime is 21213559. The next prime is 21213583. The reversal of 21213581 is 18531212.

It is a happy number.

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

It can be written as a sum of positive squares in 2 ways, for example, as 8497225 + 12716356 = 2915^2 + 3566^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21213581 is a prime.

It is a super-2 number, since 2×21213581² = 900032037687122, which contains 22 as substring.

It is a Duffinian number.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21213583) 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, 145226 + ... + 145371.

It is an arithmetic number, because the mean of its divisors is an integer number (5376063).

Almost surely, 2^21213581 is an apocalyptic number.

It is an amenable number.

21213581 is a deficient number, since it is larger than the sum of its proper divisors (290671).

21213581 is an equidigital number, since it uses as much as digits as its factorization.

21213581 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 290670.

The product of its digits is 480, while the sum is 23.

The square root of 21213581 is about 4605.8203395269. The cubic root of 21213581 is about 276.8245879917.

Adding to 21213581 its reverse (18531212), we get a palindrome (39744793).

The spelling of 21213581 in words is "twenty-one million, two hundred thirteen thousand, five hundred eighty-one".