• 721 can be written using four 4's:

721 has 4 divisors (see below), whose sum is σ = 832. Its totient is φ = 612.

The previous prime is 719. The next prime is 727. The reversal of 721 is 127.

721 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 721 - 2^{1} = 719 is a prime.

It is an alternating number because its digits alternate between odd and even.

It is a Duffinian number.

721 is a modest number, since divided by 21 gives 7 as remainder.

It is a nialpdrome in base 9, base 10 and base 15.

It is a junction number, because it is equal to *n*+sod(*n*) for *n* = 698 and 707.

It is a congruent number.

It is a panconsummate number.

It is not an unprimeable number, because it can be changed into a prime (727) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (5) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 45 + ... + 58.

It is an arithmetic number, because the mean of its divisors is an integer number (208).

721 is the 16-th hex number.

It is an amenable number.

721 is a deficient number, since it is larger than the sum of its proper divisors (111).

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

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

The sum of its prime factors is 110.

The product of its digits is 14, while the sum is 10.

The square root of 721 is about 26.8514431642. The cubic root of 721 is about 8.9669570224.

Subtracting from 721 its product of digits (14), we obtain a palindrome (707).

Adding to 721 its reverse (127), we get a palindrome (848).

It can be divided in two parts, 7 and 21, that added together give a triangular number (28 = T_{7}).

The spelling of 721 in words is "seven hundred twenty-one", and thus it is an aban number and an iban number.

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