1218115721 has 2 divisors, whose sum is σ = 1218115722. Its totient is φ = 1218115720.

The previous prime is 1218115709. The next prime is 1218115751. The reversal of 1218115721 is 1275118121.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 1206589696 + 11526025 = 34736^2 + 3395^2 .

It is an emirp because it is prime and its reverse (1275118121) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 1218115721 - 2¹⁰ = 1218114697 is a prime.

It is not a weakly prime, because it can be changed into another prime (1218115751) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 609057860 + 609057861.

It is an arithmetic number, because the mean of its divisors is an integer number (609057861).

Almost surely, 2^1218115721 is an apocalyptic number.

It is an amenable number.

1218115721 is a deficient number, since it is larger than the sum of its proper divisors (1).

1218115721 is an equidigital number, since it uses as much as digits as its factorization.

1218115721 is an evil number, because the sum of its binary digits is even.

The product of its digits is 1120, while the sum is 29.

The square root of 1218115721 is about 34901.5145946419. The cubic root of 1218115721 is about 1067.9793351802.

The spelling of 1218115721 in words is "one billion, two hundred eighteen million, one hundred fifteen thousand, seven hundred twenty-one".