1720400021 has 2 divisors, whose sum is σ = 1720400022. Its totient is φ = 1720400020.

The previous prime is 1720399999. The next prime is 1720400093. The reversal of 1720400021 is 1200040271.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 1673628100 + 46771921 = 40910^2 + 6839^2 .

It is a cyclic number.

It is not a de Polignac number, because 1720400021 - 2³⁰ = 646658197 is a prime.

It is a Sophie Germain prime.

It is a Curzon number.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (1720406021) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 860200010 + 860200011.

It is an arithmetic number, because the mean of its divisors is an integer number (860200011).

Almost surely, 2^1720400021 is an apocalyptic number.

It is an amenable number.

1720400021 is a deficient number, since it is larger than the sum of its proper divisors (1).

1720400021 is an equidigital number, since it uses as much as digits as its factorization.

1720400021 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 112, while the sum is 17.

The square root of 1720400021 is about 41477.7051076841. The cubic root of 1720400021 is about 1198.2381601347.

Adding to 1720400021 its reverse (1200040271), we get a palindrome (2920440292).

The spelling of 1720400021 in words is "one billion, seven hundred twenty million, four hundred thousand, twenty-one".