77821 has 4 divisors (see below), whose sum is σ = 79200. Its totient is φ = 76444.

The previous prime is 77813. The next prime is 77839. The reversal of 77821 is 12877.

It is a happy number.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also an emirpimes, since its reverse is a distinct semiprime: 12877 = 79 ⋅163.

It is a cyclic number.

It is not a de Polignac number, because 77821 - 2³ = 77813 is a prime.

It is a super-2 number, since 2×77821² = 12112216082, which contains 22 as substring.

It is a Duffinian number.

77821 is a lucky number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (77801) 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 3 ways as a sum of consecutive naturals, for example, 601 + ... + 718.

It is an arithmetic number, because the mean of its divisors is an integer number (19800).

2⁷⁷⁸²¹ is an apocalyptic number.

It is an amenable number.

77821 is a deficient number, since it is larger than the sum of its proper divisors (1379).

77821 is a wasteful number, since it uses less digits than its factorization.

77821 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1378.

The product of its digits is 784, while the sum is 25.

The square root of 77821 is about 278.9641554035. The cubic root of 77821 is about 42.6938777702.

It can be divided in two parts, 77 and 821, that added together give a palindrome (898).

The spelling of 77821 in words is "seventy-seven thousand, eight hundred twenty-one".