991290821 has 4 divisors (see below), whose sum is σ = 1001510412. Its totient is φ = 981071232.

The previous prime is 991290793. The next prime is 991290841. The reversal of 991290821 is 128092199.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 128092199 = 9371 ⋅13669.

It can be written as a sum of positive squares in 2 ways, for example, as 222337921 + 768952900 = 14911^2 + 27730^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-991290821 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (991290841) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 5109650 + ... + 5109843.

It is an arithmetic number, because the mean of its divisors is an integer number (250377603).

Almost surely, 2^991290821 is an apocalyptic number.

It is an amenable number.

991290821 is a deficient number, since it is larger than the sum of its proper divisors (10219591).

991290821 is a wasteful number, since it uses less digits than its factorization.

991290821 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 10219590.

The product of its (nonzero) digits is 23328, while the sum is 41.

The square root of 991290821 is about 31484.7712553228. The cubic root of 991290821 is about 997.0884715625.

The spelling of 991290821 in words is "nine hundred ninety-one million, two hundred ninety thousand, eight hundred twenty-one".