781661 has 2 divisors, whose sum is σ = 781662. Its totient is φ = 781660.

The previous prime is 781633. The next prime is 781673. The reversal of 781661 is 166187.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 525625 + 256036 = 725^2 + 506^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-781661 is a prime.

It is a super-2 number, since 2×781661² = 1221987837842, which contains 22 as substring.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (781601) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 390830 + 390831.

It is an arithmetic number, because the mean of its divisors is an integer number (390831).

2⁷⁸¹⁶⁶¹ is an apocalyptic number.

It is an amenable number.

781661 is a deficient number, since it is larger than the sum of its proper divisors (1).

781661 is an equidigital number, since it uses as much as digits as its factorization.

781661 is an evil number, because the sum of its binary digits is even.

The product of its digits is 2016, while the sum is 29.

The square root of 781661 is about 884.1159426229. The cubic root of 781661 is about 92.1169354287.

It can be divided in two parts, 7816 and 61, that multiplied together give a triangular number (476776 = T₉₇₆).

The spelling of 781661 in words is "seven hundred eighty-one thousand, six hundred sixty-one".