21651981 has 16 divisors (see below), whose sum is σ = 32094720. Its totient is φ = 12893760.

The previous prime is 21651979. The next prime is 21651989. The reversal of 21651981 is 18915612.

It is not a de Polignac number, because 21651981 - 2¹ = 21651979 is a prime.

It is a super-2 number, since 2×21651981² = 937616562448722, which contains 22 as substring.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21651989) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 7746 + ... + 10163.

It is an arithmetic number, because the mean of its divisors is an integer number (2005920).

Almost surely, 2^21651981 is an apocalyptic number.

It is an amenable number.

21651981 is a deficient number, since it is larger than the sum of its proper divisors (10442739).

21651981 is a wasteful number, since it uses less digits than its factorization.

21651981 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 17956.

The product of its digits is 4320, while the sum is 33.

The square root of 21651981 is about 4653.1689202091. The cubic root of 21651981 is about 278.7185532973.

The spelling of 21651981 in words is "twenty-one million, six hundred fifty-one thousand, nine hundred eighty-one".