661215521 has 2 divisors, whose sum is σ = 661215522. Its totient is φ = 661215520.

The previous prime is 661215517. The next prime is 661215613. The reversal of 661215521 is 125512166.

It is a happy number.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 661055521 + 160000 = 25711^2 + 400^2 .

It is a cyclic number.

It is not a de Polignac number, because 661215521 - 2² = 661215517 is a prime.

It is a super-2 number, since 2×661215521² = 874411930422602882, which contains 22 as substring.

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 (661215421) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 330607760 + 330607761.

It is an arithmetic number, because the mean of its divisors is an integer number (330607761).

Almost surely, 2^661215521 is an apocalyptic number.

It is an amenable number.

661215521 is a deficient number, since it is larger than the sum of its proper divisors (1).

661215521 is an equidigital number, since it uses as much as digits as its factorization.

661215521 is an evil number, because the sum of its binary digits is even.

The product of its digits is 3600, while the sum is 29.

The square root of 661215521 is about 25714.1113204404. Note that the first 3 decimals coincide. The cubic root of 661215521 is about 871.1929383039.

Adding to 661215521 its reverse (125512166), we get a palindrome (786727687).

The spelling of 661215521 in words is "six hundred sixty-one million, two hundred fifteen thousand, five hundred twenty-one".