78191921 has 2 divisors, whose sum is σ = 78191922. Its totient is φ = 78191920.

The previous prime is 78191899. The next prime is 78191923. The reversal of 78191921 is 12919187.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 75881521 + 2310400 = 8711^2 + 1520^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-78191921 is a prime.

It is a super-2 number, since 2×78191921² = 12227953019340482, which contains 22 as substring.

Together with 78191923, it forms a pair of twin primes.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (78191923) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 39095960 + 39095961.

It is an arithmetic number, because the mean of its divisors is an integer number (39095961).

Almost surely, 2^78191921 is an apocalyptic number.

It is an amenable number.

78191921 is a deficient number, since it is larger than the sum of its proper divisors (1).

78191921 is an equidigital number, since it uses as much as digits as its factorization.

78191921 is an evil number, because the sum of its binary digits is even.

The product of its digits is 9072, while the sum is 38.

The square root of 78191921 is about 8842.6195779305. The cubic root of 78191921 is about 427.6160140204.

The spelling of 78191921 in words is "seventy-eight million, one hundred ninety-one thousand, nine hundred twenty-one".