21621055681 has 27 divisors (see below), whose sum is σ = 23359103523.
Its totient is φ = 20009339280.
The previous prime is 21621055669. The next prime is 21621055693. The reversal of 21621055681 is 18655012612.
The square root of 21621055681 is 147041.
It is a perfect power (a square), and thus also a powerful number.
It is an interprime number because it is at equal distance from previous prime (21621055669) and next prime (21621055693).
It can be written as a sum of positive squares in only one way, i.e., 15069772081 + 6551283600 = 122759^2 + 80940^2
It is not a de Polignac number, because 21621055681 - 219 = 21620531393 is a prime.
It is a super-2 number, since 2×216210556812 (a number of 21 digits) contains 22 as substring.
It is a Duffinian number.
It is a self number, because there is not a number n which added to its sum of digits gives 21621055681.
It is not an unprimeable number, because it can be changed into a prime (21621052681) 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 in 26 ways as a sum of consecutive naturals, for example, 198358255 + ... + 198358363.
Almost surely, 221621055681 is an apocalyptic number.
21621055681 is the 147041-st square number.
21621055681 is the 73521-st centered octagonal number.
It is an amenable number.
21621055681 is a deficient number, since it is larger than the sum of its proper divisors (1738047842).
21621055681 is an frugal number, since it uses more digits than its factorization.
21621055681 is an odious number, because the sum of its binary digits is odd.
The sum of its prime factors is 398 (or 199 counting only the distinct ones).
The product of its (nonzero) digits is 28800, while the sum is 37.
The spelling of 21621055681 in words is "twenty-one billion, six hundred twenty-one million, fifty-five thousand, six hundred eighty-one".