19978921 has 2 divisors, whose sum is σ = 19978922. Its totient is φ = 19978920.

The previous prime is 19978919. The next prime is 19978939. The reversal of 19978921 is 12987991.

It is a happy number.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 19900521 + 78400 = 4461^2 + 280^2 .

It is a cyclic number.

It is not a de Polignac number, because 19978921 - 2¹ = 19978919 is a prime.

Together with 19978919, it forms a pair of twin primes.

It is not a weakly prime, because it can be changed into another prime (19978991) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 9989460 + 9989461.

It is an arithmetic number, because the mean of its divisors is an integer number (9989461).

Almost surely, 2^19978921 is an apocalyptic number.

It is an amenable number.

19978921 is a deficient number, since it is larger than the sum of its proper divisors (1).

19978921 is an equidigital number, since it uses as much as digits as its factorization.

19978921 is an evil number, because the sum of its binary digits is even.

The product of its digits is 81648, while the sum is 46.

The square root of 19978921 is about 4469.7786298652. The cubic root of 19978921 is about 271.3463661227.

The spelling of 19978921 in words is "nineteen million, nine hundred seventy-eight thousand, nine hundred twenty-one".