77820021 has 36 divisors (see below), whose sum is σ = 121044300. Its totient is φ = 49968576.

The previous prime is 77820019. The next prime is 77820023. The reversal of 77820021 is 12002877.

It is an interprime number because it is at equal distance from previous prime (77820019) and next prime (77820023).

It can be written as a sum of positive squares in 2 ways, for example, as 57972996 + 19847025 = 7614^2 + 4455^2 .

It is not a de Polignac number, because 77820021 - 2¹ = 77820019 is a prime.

It is a hoax number, since the sum of its digits (27) coincides with the sum of the digits of its distinct prime factors.

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

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (77820023) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 35 ways as a sum of consecutive naturals, for example, 190065 + ... + 190473.

Almost surely, 2^77820021 is an apocalyptic number.

It is an amenable number.

77820021 is a deficient number, since it is larger than the sum of its proper divisors (43224279).

77820021 is an frugal number, since it uses more digits than its factorization.

77820021 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 462 (or 441 counting only the distinct ones).

The product of its (nonzero) digits is 1568, while the sum is 27.

The square root of 77820021 is about 8821.5656773614. The cubic root of 77820021 is about 426.9369873762.

Adding to 77820021 its reverse (12002877), we get a palindrome (89822898).

The spelling of 77820021 in words is "seventy-seven million, eight hundred twenty thousand, twenty-one".