21251681 has 8 divisors (see below), whose sum is σ = 23409984. Its totient is φ = 19131120.

The previous prime is 21251669. The next prime is 21251689. The reversal of 21251681 is 18615212.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 21251681 - 2⁶ = 21251617 is a prime.

It is a super-2 number, since 2×21251681² = 903267890651522, which contains 22 as substring.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21251681.

It is not an unprimeable number, because it can be changed into a prime (21251689) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 8246 + ... + 10511.

It is an arithmetic number, because the mean of its divisors is an integer number (2926248).

Almost surely, 2^21251681 is an apocalyptic number.

It is an amenable number.

21251681 is a deficient number, since it is larger than the sum of its proper divisors (2158303).

21251681 is a wasteful number, since it uses less digits than its factorization.

21251681 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 18871.

The product of its digits is 960, while the sum is 26.

The square root of 21251681 is about 4609.9545550905. The cubic root of 21251681 is about 276.9902163044.

Adding to 21251681 its reverse (18615212), we get a palindrome (39866893).

The spelling of 21251681 in words is "twenty-one million, two hundred fifty-one thousand, six hundred eighty-one".