20997557 has 4 divisors (see below), whose sum is σ = 23997216. Its totient is φ = 17997900.

The previous prime is 20997553. The next prime is 20997583. The reversal of 20997557 is 75579902.

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: 75579902 = 2 ⋅37789951.

It is a cyclic number.

It is not a de Polignac number, because 20997557 - 2² = 20997553 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 (20997553) 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, 1499819 + ... + 1499832.

It is an arithmetic number, because the mean of its divisors is an integer number (5999304).

Almost surely, 2^20997557 is an apocalyptic number.

It is an amenable number.

20997557 is a deficient number, since it is larger than the sum of its proper divisors (2999659).

20997557 is an equidigital number, since it uses as much as digits as its factorization.

20997557 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2999658.

The product of its (nonzero) digits is 198450, while the sum is 44.

The square root of 20997557 is about 4582.3091340502. The cubic root of 20997557 is about 275.8817187284.

The spelling of 20997557 in words is "twenty million, nine hundred ninety-seven thousand, five hundred fifty-seven".