Journal of Integer Sequences, Vol. 15 (2012), Article 12.6.1

## On Lower Order Extremal Integral Sets Avoiding Prime Pairwise Sums

### Ram Krishna Pandey Department of Mathematics Indian Institute of Technology, Patna Patliputra Colony, Patna - 800013 India

Abstract:

Let A be a subset of {1,2, ..., n} such that the sum of no two distinct elements of A is a prime number. Such a subset is called a prime-sumset-free subset of {1,2, ..., n}. A prime-sumset-free subset is called an extremal prime-sumset-free subset of {1,2, ..., n} if A ∪ {a} is not a prime-sumset-free subset for any a ∈ {1,2, ..., n} \ A. We prove that if n ≥ 10 then there is no any extremal prime-sumset-free subset of {1,2, ..., n} of order 2 and if n ≥ 13 then there is no any extremal prime-sumset-free subset of {1,2, ..., n} of order 3. Moreover, we prove that for each integer k ≥ 2, there exists a nk such that if nnk then there does not exist any extremal prime-sumset-free subset of {1,2, ..., n} of length k. Furthermore, for some small values of n, we give the orders of all extremal prime-sumset-free subset of {1,2, ..., n}, along with an example of each order and we give all extremal prime-sumset-free subsets of {1,2, ..., n} of orders 2 and 3 for n ≤ 13.

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Received March 16 2012; revised version received June 1 2012. Published in Journal of Integer Sequences, June 12 2012.

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