661391 has 4 divisors (see below), whose sum is σ = 663384. Its totient is φ = 659400.

The previous prime is 661373. The next prime is 661393. The reversal of 661391 is 193166.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 661391 - 2⁶ = 661327 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 661391.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (661393) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 365 + ... + 1206.

It is an arithmetic number, because the mean of its divisors is an integer number (165846).

2⁶⁶¹³⁹¹ is an apocalyptic number.

661391 is a deficient number, since it is larger than the sum of its proper divisors (1993).

661391 is a wasteful number, since it uses less digits than its factorization.

661391 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1992.

The product of its digits is 972, while the sum is 26.

The square root of 661391 is about 813.2594911835. The cubic root of 661391 is about 87.1269999685.

It can be divided in two parts, 66 and 1391, that multiplied together give a triangular number (91806 = T₄₂₈).

The spelling of 661391 in words is "six hundred sixty-one thousand, three hundred ninety-one".