20400991 has 4 divisors (see below), whose sum is σ = 21970312. Its totient is φ = 18831672.

The previous prime is 20400967. The next prime is 20400997. The reversal of 20400991 is 19900402.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 19900402 = 2 ⋅9950201.

It is a cyclic number.

It is not a de Polignac number, because 20400991 - 2⁵ = 20400959 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (20400997) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 784641 + ... + 784666.

It is an arithmetic number, because the mean of its divisors is an integer number (5492578).

Almost surely, 2^20400991 is an apocalyptic number.

20400991 is a deficient number, since it is larger than the sum of its proper divisors (1569321).

20400991 is a wasteful number, since it uses less digits than its factorization.

20400991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1569320.

The product of its (nonzero) digits is 648, while the sum is 25.

The square root of 20400991 is about 4516.7456204661. The cubic root of 20400991 is about 273.2438660523.

The spelling of 20400991 in words is "twenty million, four hundred thousand, nine hundred ninety-one".