20991555 has 56 divisors (see below), whose sum is σ = 40764528. Its totient is φ = 10310976.

The previous prime is 20991533. The next prime is 20991577. The reversal of 20991555 is 55519902.

It is an interprime number because it is at equal distance from previous prime (20991533) and next prime (20991577).

It is not a de Polignac number, because 20991555 - 2⁷ = 20991427 is a prime.

It is an unprimeable number.

It is a polite number, since it can be written in 55 ways as a sum of consecutive naturals, for example, 47164 + ... + 47606.

It is an arithmetic number, because the mean of its divisors is an integer number (727938).

Almost surely, 2^20991555 is an apocalyptic number.

20991555 is a deficient number, since it is larger than the sum of its proper divisors (19772973).

20991555 is an equidigital number, since it uses as much as digits as its factorization.

20991555 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 479 (or 464 counting only the distinct ones).

The product of its (nonzero) digits is 20250, while the sum is 36.

The square root of 20991555 is about 4581.6541772596. The cubic root of 20991555 is about 275.8554299579.

The spelling of 20991555 in words is "twenty million, nine hundred ninety-one thousand, five hundred fifty-five".