20978191 has 4 divisors (see below), whose sum is σ = 22591912. Its totient is φ = 19364472.

The previous prime is 20978189. The next prime is 20978197. The reversal of 20978191 is 19187902.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 19187902 = 2 ⋅9593951.

It is a cyclic number.

It is not a de Polignac number, because 20978191 - 2¹ = 20978189 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 20978191.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (20978197) 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, 806841 + ... + 806866.

It is an arithmetic number, because the mean of its divisors is an integer number (5647978).

Almost surely, 2^20978191 is an apocalyptic number.

20978191 is a deficient number, since it is larger than the sum of its proper divisors (1613721).

20978191 is a wasteful number, since it uses less digits than its factorization.

20978191 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1613720.

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

The square root of 20978191 is about 4580.1955198441. The cubic root of 20978191 is about 275.7968776087.

It can be divided in two parts, 209 and 78191, that added together give a square (78400 = 280²).

The spelling of 20978191 in words is "twenty million, nine hundred seventy-eight thousand, one hundred ninety-one".