21978631 has 2 divisors, whose sum is σ = 21978632. Its totient is φ = 21978630.

The previous prime is 21978617. The next prime is 21978641. The reversal of 21978631 is 13687912.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21978631 - 2¹¹ = 21976583 is a prime.

It is a super-2 number, since 2×21978631² = 966120441268322, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21978601) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10989315 + 10989316.

It is an arithmetic number, because the mean of its divisors is an integer number (10989316).

Almost surely, 2^21978631 is an apocalyptic number.

21978631 is a deficient number, since it is larger than the sum of its proper divisors (1).

21978631 is an equidigital number, since it uses as much as digits as its factorization.

21978631 is an evil number, because the sum of its binary digits is even.

The product of its digits is 18144, while the sum is 37.

The square root of 21978631 is about 4688.1372633488. The cubic root of 21978631 is about 280.1131812846.

The spelling of 21978631 in words is "twenty-one million, nine hundred seventy-eight thousand, six hundred thirty-one".