21720821031 has 64 divisors (see below), whose sum is σ = 40276454400. Its totient is φ = 11262872448.

The previous prime is 21720821027. The next prime is 21720821123. The reversal of 21720821031 is 13012802712.

21720821031 is a `hidden beast` number, since 217 + 208 + 210 + 31 = 666.

It is not a de Polignac number, because 21720821031 - 2² = 21720821027 is a prime.

It is a super-2 number, since 2×21720821031² (a number of 21 digits) contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is a junction number, because it is equal to n+sod(n) for n = 21720820986 and 21720821004.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21720821021) by changing a digit.

It is a polite number, since it can be written in 63 ways as a sum of consecutive naturals, for example, 70045 + ... + 219881.

It is an arithmetic number, because the mean of its divisors is an integer number (629319600).

Almost surely, 2^21720821031 is an apocalyptic number.

21720821031 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

21720821031 is a deficient number, since it is larger than the sum of its proper divisors (18555633369).

21720821031 is a wasteful number, since it uses less digits than its factorization.

21720821031 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 149925 (or 149919 counting only the distinct ones).

The product of its (nonzero) digits is 1344, while the sum is 27.

The spelling of 21720821031 in words is "twenty-one billion, seven hundred twenty million, eight hundred twenty-one thousand, thirty-one".