7821721 has 2 divisors, whose sum is σ = 7821722. Its totient is φ = 7821720.

The previous prime is 7821719. The next prime is 7821733. The reversal of 7821721 is 1271287.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 4347225 + 3474496 = 2085^2 + 1864^2 .

It is a cyclic number.

It is not a de Polignac number, because 7821721 - 2¹ = 7821719 is a prime.

It is a super-2 number, since 2×7821721² = 122358638803682, which contains 22 as substring.

Together with 7821719, it forms a pair of twin primes.

It is a self number, because there is not a number n which added to its sum of digits gives 7821721.

It is not a weakly prime, because it can be changed into another prime (7821701) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 3910860 + 3910861.

It is an arithmetic number, because the mean of its divisors is an integer number (3910861).

Almost surely, 2^7821721 is an apocalyptic number.

It is an amenable number.

7821721 is a deficient number, since it is larger than the sum of its proper divisors (1).

7821721 is an equidigital number, since it uses as much as digits as its factorization.

7821721 is an evil number, because the sum of its binary digits is even.

The product of its digits is 1568, while the sum is 28.

The square root of 7821721 is about 2796.7339880654. The cubic root of 7821721 is about 198.5031670698.

The spelling of 7821721 in words is "seven million, eight hundred twenty-one thousand, seven hundred twenty-one".