1217665721 has 2 divisors, whose sum is σ = 1217665722. Its totient is φ = 1217665720.

The previous prime is 1217665679. The next prime is 1217665733. The reversal of 1217665721 is 1275667121.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 786522025 + 431143696 = 28045^2 + 20764^2 .

It is a cyclic number.

It is not a de Polignac number, because 1217665721 - 2²⁶ = 1150556857 is a prime.

It is a Sophie Germain prime.

It is a Curzon number.

It is not a weakly prime, because it can be changed into another prime (1217665321) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 608832860 + 608832861.

It is an arithmetic number, because the mean of its divisors is an integer number (608832861).

Almost surely, 2^1217665721 is an apocalyptic number.

It is an amenable number.

1217665721 is a deficient number, since it is larger than the sum of its proper divisors (1).

1217665721 is an equidigital number, since it uses as much as digits as its factorization.

1217665721 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 35280, while the sum is 38.

The square root of 1217665721 is about 34895.0672875121. The cubic root of 1217665721 is about 1067.8478069285.

The spelling of 1217665721 in words is "one billion, two hundred seventeen million, six hundred sixty-five thousand, seven hundred twenty-one".