6138991981 has 2 divisors, whose sum is σ = 6138991982. Its totient is φ = 6138991980.

The previous prime is 6138991927. The next prime is 6138992003. The reversal of 6138991981 is 1891998316.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 4161411081 + 1977580900 = 64509^2 + 44470^2 .

It is a cyclic number.

It is not a de Polignac number, because 6138991981 - 2⁷ = 6138991853 is a prime.

It is a super-2 number, since 2×6138991981² = 75374445085564608722, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (6138991921) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 3069495990 + 3069495991.

It is an arithmetic number, because the mean of its divisors is an integer number (3069495991).

Almost surely, 2^6138991981 is an apocalyptic number.

It is an amenable number.

6138991981 is a deficient number, since it is larger than the sum of its proper divisors (1).

6138991981 is an equidigital number, since it uses as much as digits as its factorization.

6138991981 is an evil number, because the sum of its binary digits is even.

The product of its digits is 839808, while the sum is 55.

The square root of 6138991981 is about 78351.7197067174. The cubic root of 6138991981 is about 1831.0450182082.

The spelling of 6138991981 in words is "six billion, one hundred thirty-eight million, nine hundred ninety-one thousand, nine hundred eighty-one".