13781 has 2 divisors, whose sum is σ = 13782.
Its totient is φ = 13780.
The previous prime is 13763. The next prime is 13789. The reversal of 13781 is 18731.
It is a strong prime.
It can be written as a sum of positive squares in only one way, i.e., 12100 + 1681 = 110^2 + 41^2
It is an emirp because it is prime and its reverse (18731) is a distict prime.
It is a cyclic number.
It is not a de Polignac number, because 13781 - 210 = 12757 is a prime.
It is a super-2 number, since 2×137812 = 379831922, which contains 22 as substring.
It is a congruent number.
It is not a weakly prime, because it can be changed into another prime (13789) by changing a digit.
It is a polite number, since it can be written as a sum of consecutive naturals, namely, 6890 + 6891.
It is an arithmetic number, because the mean of its divisors is an integer number (6891).
213781 is an apocalyptic number.
13781 is the 53-rd centered decagonal number.
It is an amenable number.
13781 is a deficient number, since it is larger than the sum of its proper divisors (1).
13781 is an equidigital number, since it uses as much as digits as its factorization.
13781 is an odious number, because the sum of its binary digits is odd.
The product of its digits is 168, while the sum is 20.
The square root of 13781 is about 117.3925040196.
The cubic root of 13781 is about 23.9750898950.
Subtracting 13781 from its reverse (18731), we obtain a triangular number (4950 = T99).
It can be divided in two parts, 13 and 781, that multiplied together give a triangular number (10153 = T142).
The spelling of 13781 in words is "thirteen thousand, seven hundred eighty-one".