21661643 has 2 divisors, whose sum is σ = 21661644. Its totient is φ = 21661642.

The previous prime is 21661639. The next prime is 21661657. The reversal of 21661643 is 34616612.

It is a happy number.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21661643 - 2² = 21661639 is a prime.

It is not a weakly prime, because it can be changed into another prime (21661693) 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 as a sum of consecutive naturals, namely, 10830821 + 10830822.

It is an arithmetic number, because the mean of its divisors is an integer number (10830822).

Almost surely, 2^21661643 is an apocalyptic number.

21661643 is a deficient number, since it is larger than the sum of its proper divisors (1).

21661643 is an equidigital number, since it uses as much as digits as its factorization.

21661643 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 5184, while the sum is 29.

The square root of 21661643 is about 4654.2070216096. The cubic root of 21661643 is about 278.7600056735.

The spelling of 21661643 in words is "twenty-one million, six hundred sixty-one thousand, six hundred forty-three".