21720821123 has 2 divisors, whose sum is σ = 21720821124. Its totient is φ = 21720821122.

The previous prime is 21720821027. The next prime is 21720821131. The reversal of 21720821123 is 32112802712.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21720821123 - 2¹⁶ = 21720755587 is a prime.

It is a super-2 number, since 2×21720821123² (a number of 21 digits) contains 22 as substring.

It is a Sophie Germain prime.

It is not a weakly prime, because it can be changed into another prime (21720821153) 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 as a sum of consecutive naturals, namely, 10860410561 + 10860410562.

It is an arithmetic number, because the mean of its divisors is an integer number (10860410562).

Almost surely, 2^21720821123 is an apocalyptic number.

21720821123 is a deficient number, since it is larger than the sum of its proper divisors (1).

21720821123 is an equidigital number, since it uses as much as digits as its factorization.

21720821123 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 2688, while the sum is 29.

The spelling of 21720821123 in words is "twenty-one billion, seven hundred twenty million, eight hundred twenty-one thousand, one hundred twenty-three".