In mathematics, a sum-free sequence is an increasing positive integer sequence
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such that for each
The definition of sum-free sequence is different of that of sum-free set, because in a sum-free set only the sums of two elements must be avoided, while a sum-free sequence must avoid sums of larger sets of elements as well.
Example
The powers of two,
1, 2, 4, 8, 16, ...form a sum-free sequence: each term in the sequence is one more than the sum of all preceding terms, and so cannot be represented as a sum of preceding terms.
Sums of reciprocals
A set of integers is said to be small if the sum of its reciprocals converges to a finite value. For instance, by the prime number theorem, the prime numbers are not small. Paul Erdős (1962) proved that every sum-free sequence is small, and asked how large the sum of reciprocals could be. For instance, the sum of the reciprocals of the powers of two (a geometric series) is two.
If
Density
It follows from the fact that sum-free sequences are small that they have zero Schnirelmann density; that is, if