1216818721 has 2 divisors, whose sum is σ = 1216818722. Its totient is φ = 1216818720.

The previous prime is 1216818709. The next prime is 1216818737. The reversal of 1216818721 is 1278186121.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 1047428496 + 169390225 = 32364^2 + 13015^2 .

It is a cyclic number.

It is not a de Polignac number, because 1216818721 - 2⁵ = 1216818689 is a prime.

It is not a weakly prime, because it can be changed into another prime (1216812721) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 608409360 + 608409361.

It is an arithmetic number, because the mean of its divisors is an integer number (608409361).

Almost surely, 2^1216818721 is an apocalyptic number.

It is an amenable number.

1216818721 is a deficient number, since it is larger than the sum of its proper divisors (1).

1216818721 is an equidigital number, since it uses as much as digits as its factorization.

1216818721 is an evil number, because the sum of its binary digits is even.

The product of its digits is 10752, while the sum is 37.

The square root of 1216818721 is about 34882.9287904557. The cubic root of 1216818721 is about 1067.6001536053.

The spelling of 1216818721 in words is "one billion, two hundred sixteen million, eight hundred eighteen thousand, seven hundred twenty-one".