391249821 has 4 divisors (see below), whose sum is σ = 521666432. Its totient is φ = 260833212.

The previous prime is 391249811. The next prime is 391249823. The reversal of 391249821 is 128942193.

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: 128942193 = 3 ⋅42980731.

It is not a de Polignac number, because 391249821 - 2⁷ = 391249693 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 391249821.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (391249823) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 65208301 + ... + 65208306.

It is an arithmetic number, because the mean of its divisors is an integer number (130416608).

Almost surely, 2^391249821 is an apocalyptic number.

It is an amenable number.

391249821 is a deficient number, since it is larger than the sum of its proper divisors (130416611).

391249821 is a wasteful number, since it uses less digits than its factorization.

391249821 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 130416610.

The product of its digits is 31104, while the sum is 39.

The square root of 391249821 is about 19780.0359200887. The cubic root of 391249821 is about 731.3939842999.

The spelling of 391249821 in words is "three hundred ninety-one million, two hundred forty-nine thousand, eight hundred twenty-one".