21645551 has 2 divisors, whose sum is σ = 21645552. Its totient is φ = 21645550.

The previous prime is 21645527. The next prime is 21645619. The reversal of 21645551 is 15554612.

It is a happy number.

It is a weak prime.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21645551 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21645511) 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, 10822775 + 10822776.

It is an arithmetic number, because the mean of its divisors is an integer number (10822776).

Almost surely, 2^21645551 is an apocalyptic number.

21645551 is a deficient number, since it is larger than the sum of its proper divisors (1).

21645551 is an equidigital number, since it uses as much as digits as its factorization.

21645551 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 6000, while the sum is 29.

The square root of 21645551 is about 4652.4779419144. The cubic root of 21645551 is about 278.6909601680.

The spelling of 21645551 in words is "twenty-one million, six hundred forty-five thousand, five hundred fifty-one".