661521 has 24 divisors (see below), whose sum is σ = 1080640. Its totient is φ = 352512.

The previous prime is 661517. The next prime is 661541. The reversal of 661521 is 125166.

It is a happy number.

It is not a de Polignac number, because 661521 - 2² = 661517 is a prime.

It is a super-2 number, since 2×661521² = 875220066882, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (21).

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (661541) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 6015 + ... + 6123.

2⁶⁶¹⁵²¹ is an apocalyptic number.

It is an amenable number.

661521 is a deficient number, since it is larger than the sum of its proper divisors (419119).

661521 is a wasteful number, since it uses less digits than its factorization.

661521 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 153 (or 136 counting only the distinct ones).

The product of its digits is 360, while the sum is 21.

The square root of 661521 is about 813.3394125456. The cubic root of 661521 is about 87.1327080232.

Adding to 661521 its reverse (125166), we get a palindrome (786687).

The spelling of 661521 in words is "six hundred sixty-one thousand, five hundred twenty-one".