21353681 has 4 divisors (see below), whose sum is σ = 21593700. Its totient is φ = 21113664.

The previous prime is 21353639. The next prime is 21353711. The reversal of 21353681 is 18635312.

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 4910656 + 16443025 = 2216^2 + 4055^2 .

It is a cyclic number.

It is not a de Polignac number, because 21353681 - 2¹⁴ = 21337297 is a prime.

It is a super-2 number, since 2×21353681² = 911959384499522, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21353611) 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, 119876 + ... + 120053.

It is an arithmetic number, because the mean of its divisors is an integer number (5398425).

Almost surely, 2^21353681 is an apocalyptic number.

It is an amenable number.

21353681 is a deficient number, since it is larger than the sum of its proper divisors (240019).

21353681 is an equidigital number, since it uses as much as digits as its factorization.

21353681 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 240018.

The product of its digits is 4320, while the sum is 29.

The square root of 21353681 is about 4621.0043280655. The cubic root of 21353681 is about 277.4326584964.

Adding to 21353681 its reverse (18635312), we get a palindrome (39988993).

The spelling of 21353681 in words is "twenty-one million, three hundred fifty-three thousand, six hundred eighty-one".