78219721 has 2 divisors, whose sum is σ = 78219722. Its totient is φ = 78219720.

The previous prime is 78219689. The next prime is 78219761. The reversal of 78219721 is 12791287.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 76370121 + 1849600 = 8739^2 + 1360^2 .

It is a cyclic number.

It is not a de Polignac number, because 78219721 - 2⁵ = 78219689 is a prime.

It is a super-2 number, since 2×78219721² = 12236649506635682, which contains 22 as substring.

It is not a weakly prime, because it can be changed into another prime (78219761) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 39109860 + 39109861.

It is an arithmetic number, because the mean of its divisors is an integer number (39109861).

Almost surely, 2^78219721 is an apocalyptic number.

It is an amenable number.

78219721 is a deficient number, since it is larger than the sum of its proper divisors (1).

78219721 is an equidigital number, since it uses as much as digits as its factorization.

78219721 is an evil number, because the sum of its binary digits is even.

The product of its digits is 14112, while the sum is 37.

The square root of 78219721 is about 8844.1913706116. The cubic root of 78219721 is about 427.6666855667. Note that the first 4 decimals are identical.

It can be divided in two parts, 782 and 19721, that added together give a triangular number (20503 = T₂₀₂).

The spelling of 78219721 in words is "seventy-eight million, two hundred nineteen thousand, seven hundred twenty-one".