77781521981 has 2 divisors, whose sum is σ = 77781521982. Its totient is φ = 77781521980.

The previous prime is 77781521977. The next prime is 77781522001. The reversal of 77781521981 is 18912518777.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 72667984900 + 5113537081 = 269570^2 + 71509^2 .

It is an emirp because it is prime and its reverse (18912518777) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 77781521981 - 2² = 77781521977 is a prime.

It is a super-2 number, since 2×77781521981² (a number of 23 digits) contains 22 as substring.

It is a Sophie Germain prime.

It is a Curzon number.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (77721521981) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 38890760990 + 38890760991.

It is an arithmetic number, because the mean of its divisors is an integer number (38890760991).

Almost surely, 2^77781521981 is an apocalyptic number.

It is an amenable number.

77781521981 is a deficient number, since it is larger than the sum of its proper divisors (1).

77781521981 is an equidigital number, since it uses as much as digits as its factorization.

77781521981 is an evil number, because the sum of its binary digits is even.

The product of its digits is 1975680, while the sum is 56.

The spelling of 77781521981 in words is "seventy-seven billion, seven hundred eighty-one million, five hundred twenty-one thousand, nine hundred eighty-one".