1287991 has 4 divisors (see below), whose sum is σ = 1355800. Its totient is φ = 1220184.

The previous prime is 1287989. The next prime is 1287997. The reversal of 1287991 is 1997821.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 1287991 - 2¹ = 1287989 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 1287991.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1287997) 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, 33876 + ... + 33913.

It is an arithmetic number, because the mean of its divisors is an integer number (338950).

2^1287991 is an apocalyptic number.

1287991 is a deficient number, since it is larger than the sum of its proper divisors (67809).

1287991 is an equidigital number, since it uses as much as digits as its factorization.

1287991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 67808.

The product of its digits is 9072, while the sum is 37.

The square root of 1287991 is about 1134.8969116180. The cubic root of 1287991 is about 108.8021830859.

It can be divided in two parts, 1287 and 991, that added together give a triangular number (2278 = T₆₇).

The spelling of 1287991 in words is "one million, two hundred eighty-seven thousand, nine hundred ninety-one".