21691643 has 4 divisors (see below), whose sum is σ = 22967640. Its totient is φ = 20415648.

The previous prime is 21691633. The next prime is 21691693. The reversal of 21691643 is 34619612.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 21691643 - 2⁴ = 21691627 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21691613) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 637973 + ... + 638006.

It is an arithmetic number, because the mean of its divisors is an integer number (5741910).

Almost surely, 2^21691643 is an apocalyptic number.

21691643 is a deficient number, since it is larger than the sum of its proper divisors (1275997).

21691643 is a wasteful number, since it uses less digits than its factorization.

21691643 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1275996.

The product of its digits is 7776, while the sum is 32.

The square root of 21691643 is about 4657.4287970940. The cubic root of 21691643 is about 278.8886346129. Note that the first 3 decimals are identical.

The spelling of 21691643 in words is "twenty-one million, six hundred ninety-one thousand, six hundred forty-three".