1221991 has 4 divisors (see below), whose sum is σ = 1228392. Its totient is φ = 1215592.

The previous prime is 1221971. The next prime is 1222003. The reversal of 1221991 is 1991221.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 1991221 = 73 ⋅27277.

It is a cyclic number.

It is not a de Polignac number, because 1221991 - 2⁵ = 1221959 is a prime.

It is a Duffinian number.

1221991 is a lucky number.

It is a nialpdrome in base 11.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1221971) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2905 + ... + 3298.

It is an arithmetic number, because the mean of its divisors is an integer number (307098).

2^1221991 is an apocalyptic number.

1221991 is a deficient number, since it is larger than the sum of its proper divisors (6401).

1221991 is an equidigital number, since it uses as much as digits as its factorization.

1221991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 6400.

The product of its digits is 324, while the sum is 25.

The square root of 1221991 is about 1105.4370176541. The cubic root of 1221991 is about 106.9110682727.

The spelling of 1221991 in words is "one million, two hundred twenty-one thousand, nine hundred ninety-one".