21909981 has 8 divisors (see below), whose sum is σ = 29481760. Its totient is φ = 14472432.

The previous prime is 21909977. The next prime is 21909989. The reversal of 21909981 is 18990912.

It is a happy number.

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

It is not a de Polignac number, because 21909981 - 2² = 21909977 is a prime.

It is a super-2 number, since 2×21909981² = 960094534840722, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21909989) 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, 33175 + ... + 33828.

It is an arithmetic number, because the mean of its divisors is an integer number (3685220).

Almost surely, 2^21909981 is an apocalyptic number.

It is an amenable number.

21909981 is a deficient number, since it is larger than the sum of its proper divisors (7571779).

21909981 is a wasteful number, since it uses less digits than its factorization.

21909981 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 67115.

The product of its (nonzero) digits is 11664, while the sum is 39.

The square root of 21909981 is about 4680.8098658245. The cubic root of 21909981 is about 279.8212336798.

The spelling of 21909981 in words is "twenty-one million, nine hundred nine thousand, nine hundred eighty-one".