991557121 has 3 divisors (see below), whose sum is σ = 991588611. Its totient is φ = 991525632.

The previous prime is 991557109. The next prime is 991557137. The reversal of 991557121 is 121755199.

The square root of 991557121 is 31489.

It is a perfect power (a square), and thus also a powerful number.

991557121 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It can be written as a sum of positive squares in only one way, i.e., 589955521 + 401601600 = 24289^2 + 20040^2 .

It is not a de Polignac number, because 991557121 - 2⁵ = 991557089 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (991557221) by changing a digit.

It is a polite number, since it can be written in 2 ways as a sum of consecutive naturals, for example, 15745 + ... + 47233.

It is an arithmetic number, because the mean of its divisors is an integer number (330529537).

Almost surely, 2^991557121 is an apocalyptic number.

991557121 is the 31489-th square number.

991557121 is the 15745-th centered octagonal number.

It is an amenable number.

991557121 is a deficient number, since it is larger than the sum of its proper divisors (31490).

991557121 is an frugal number, since it uses more digits than its factorization.

991557121 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 62978 (or 31489 counting only the distinct ones).

The product of its digits is 28350, while the sum is 40.

The cubic root of 991557121 is about 997.1777493947.

It can be divided in two parts, 99155 and 7121, that added together give a square (106276 = 326²).

The spelling of 991557121 in words is "nine hundred ninety-one million, five hundred fifty-seven thousand, one hundred twenty-one".