21653991 has 12 divisors (see below), whose sum is σ = 32123832. Its totient is φ = 14045616.

The previous prime is 21653987. The next prime is 21653993. The reversal of 21653991 is 19935612.

It is not a de Polignac number, because 21653991 - 2² = 21653987 is a prime.

It is a Smith number, since the sum of its digits (36) coincides with the sum of the digits of its prime factors.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21653993) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 32181 + ... + 32846.

It is an arithmetic number, because the mean of its divisors is an integer number (2676986).

Almost surely, 2^21653991 is an apocalyptic number.

21653991 is a deficient number, since it is larger than the sum of its proper divisors (10469841).

21653991 is a wasteful number, since it uses less digits than its factorization.

21653991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 65070 (or 65067 counting only the distinct ones).

The product of its digits is 14580, while the sum is 36.

The square root of 21653991 is about 4653.3848970400. The cubic root of 21653991 is about 278.7271777115.

The spelling of 21653991 in words is "twenty-one million, six hundred fifty-three thousand, nine hundred ninety-one".