Somos sequence

For an integer number k larger than 1, the Somos-k sequence ${\displaystyle (a_{0},a_{1},a_{2},\ldots )}$ is defined by the equation

${\displaystyle a_{n}a_{n-k}=a_{n-1}a_{n-k+1}+a_{n-2}a_{n-k+2}+\cdots +a_{n-(k-1)/2}a_{n-(k+1)/2}}$

when k is odd, or by the analogous equation

${\displaystyle a_{n}a_{n-k}=a_{n-1}a_{n-k+1}+a_{n-2}a_{n-k+2}+\cdots +(a_{n-k/2})^{2}}$

when k is even, together with the initial values

a_i = 1 for i < k.

For k = 2 or 3, these recursions are very simple (there is no addition on the right-hand side) and they define the all-ones sequence (1, 1, 1, 1, 1, 1, ...). In the first nontrivial case, k = 4, the defining equation is

${\displaystyle a_{n}a_{n-4}=a_{n-1}a_{n-3}+a_{n-2}^{2}}$

while for k = 5 the equation is

${\displaystyle a_{n}a_{n-5}=a_{n-1}a_{n-4}+a_{n-2}a_{n-3}\,.}$

These equations can be rearranged into the form of a recurrence relation, in which the value a_n on the left hand side of the recurrence is defined by a formula on the right hand side, by dividing the formula by a_n − k. For k = 4, this yields the recurrence

${\displaystyle a_{n}={\frac {a_{n-1}a_{n-3}+a_{n-2}^{2}}{a_{n-4}}}}$

while for k = 5 it gives the recurrence

${\displaystyle a_{n}={\frac {a_{n-1}a_{n-4}+a_{n-2}a_{n-3}}{a_{n-5}}}.}$

While in the usual definition of the Somos sequences, the values of a_i for i < k are all set equal to 1, it is also possible to define other sequences by using the same recurrences with different initial values.

The values in the Somos-4 sequence are

1, 1, 1, 1, 2, 3, 7, 23, 59, 314, 1529, 8209, 83313, 620297, 7869898, ... (sequence A006720 in the OEIS).

The values in the Somos-5 sequence are

1, 1, 1, 1, 1, 2, 3, 5, 11, 37, 83, 274, 1217, 6161, 22833, 165713, ... (sequence A006721 in the OEIS).

The values in the Somos-6 sequence are

1, 1, 1, 1, 1, 1, 3, 5, 9, 23, 75, 421, 1103, 5047, 41783, 281527, ... (sequence A006722 in the OEIS).

The values in the Somos-7 sequence are

1, 1, 1, 1, 1, 1, 1, 3, 5, 9, 17, 41, 137, 769, 1925, 7203, 34081, ... (sequence A006723 in the OEIS).

The form of the recurrences describing the Somos sequences involves divisions, making it appear likely that the sequences defined by these recurrence will contain fractional values. Nevertheless, for k ≤ 7 the Somos sequences contain only integer values. Several mathematicians have studied the problem of proving and explaining this integer property of the Somos sequences; it is closely related to the combinatorics of cluster algebras.[1][2][3]

For k ≥ 8 the analogously defined sequences eventually contain fractional values. For Somos-8 the first fractional value is the 19th term with value 420514/7.

For k < 7, changing the initial values (but using the same recurrence relation) also typically results in fractional values.

• Jim Propp's Somos Sequence Site
• Weisstein, Eric W. "Somos Sequence". MathWorld.
• The Troublemaker Number. Numberphile video on the Somos sequences