21208817549 has 2 divisors, whose sum is σ = 21208817550. Its totient is φ = 21208817548.

The previous prime is 21208817543. The next prime is 21208817587. The reversal of 21208817549 is 94571880212.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 19420373449 + 1788444100 = 139357^2 + 42290^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21208817549 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21208817543) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10604408774 + 10604408775.

It is an arithmetic number, because the mean of its divisors is an integer number (10604408775).

Almost surely, 2^21208817549 is an apocalyptic number.

It is an amenable number.

21208817549 is a deficient number, since it is larger than the sum of its proper divisors (1).

21208817549 is an equidigital number, since it uses as much as digits as its factorization.

21208817549 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 322560, while the sum is 47.

The spelling of 21208817549 in words is "twenty-one billion, two hundred eight million, eight hundred seventeen thousand, five hundred forty-nine".