21612781 has 8 divisors (see below), whose sum is σ = 22292928. Its totient is φ = 20940000.

The previous prime is 21612769. The next prime is 21612827. The reversal of 21612781 is 18721612.

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 21612781 - 2⁵ = 21612749 is a prime.

It is a super-2 number, since 2×21612781² = 934224605107922, which contains 22 as substring.

It is an alternating number because its digits alternate between even and odd.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21612751) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 4446 + ... + 7936.

It is an arithmetic number, because the mean of its divisors is an integer number (2786616).

Almost surely, 2^21612781 is an apocalyptic number.

It is an amenable number.

21612781 is a deficient number, since it is larger than the sum of its proper divisors (680147).

21612781 is a wasteful number, since it uses less digits than its factorization.

21612781 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3683.

The product of its digits is 1344, while the sum is 28.

The square root of 21612781 is about 4648.9548287760. The cubic root of 21612781 is about 278.5502489517.

The spelling of 21612781 in words is "twenty-one million, six hundred twelve thousand, seven hundred eighty-one".