1651 has 4 divisors (see below), whose sum is σ = 1792. Its totient is φ = 1512.

The previous prime is 1637. The next prime is 1657. The reversal of 1651 is 1561.

1651 = T₉ + T₁₀ + ... + T₂₁.

1651 = 14² + 15² + ... + 19².

1651 is nontrivially palindromic in base 2.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 1561 = 7 ⋅223.

It is a cyclic number.

It is not a de Polignac number, because 1651 - 2⁵ = 1619 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (13), and also a Moran number because the ratio is a prime number: 127 = 1651 / (1 + 6 + 5 + 1).

It is a Duffinian number.

It is a plaindrome in base 9.

It is a nialpdrome in base 15.

It is a zygodrome in base 2.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1657) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 51 + ... + 76.

It is an arithmetic number, because the mean of its divisors is an integer number (448).

2¹⁶⁵¹ is an apocalyptic number.

1651 is the 26-th heptagonal number.

1651 is a deficient number, since it is larger than the sum of its proper divisors (141).

1651 is a wasteful number, since it uses less digits than its factorization.

1651 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 140.

The product of its digits is 30, while the sum is 13.

The square root of 1651 is about 40.6324993078. The cubic root of 1651 is about 11.8190442261.

Adding to 1651 its product of digits (30), we get a square (1681 = 41²).

The spelling of 1651 in words is "one thousand, six hundred fifty-one".