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A195854 Smallest possible largest number in a set of n integers such that sum of two elements is always a perfect square. 2
1, 5, 94, 31122, 447422978 (list; graph; refs; listen; history; text; internal format)
OFFSET

2,2

COMMENTS

Note that in the examples, first term is negative (since we are asking the smallest) and the remaining terms are positive (since perfect squares cannot be negative). Solution for n=7 is not found.

Let g(N) denote the greatest integer g such that there is a set Q such that there is a set A that is a subset of {1, 2, 3, 4, 5 ...} such that the cardinality equals g. Then, there is an efficiently computable real number K such that if N exceeds K, then g(N) < 37 * log(N). The proof of this is based on a sieve result called Gallagher's larger sieve.

In any n-tuplet having the property that sum of any two numbers is a perfect square, at least one of the number must be even. It should also not have odd numbers of the form 4k + 1 or 4k + 3, since perfect squares are only 0 or 1 (mod 4).

REFERENCES

Richard K. Guy, Unsolved Problems in Number Theory, Springer, Third Edition, 2004, p. 268-270.

LINKS

Table of n, a(n) for n=2..6.

Erich Friedman, Problem of the month December 2010

Allen J. MacLeod, On Set of Integers where Each Pair Sums to a Square

J. Rivat, A. Sárközy, and C. L. Stewart, Congruence properties on the Omega-function on sumsets, Illinois J. Math. 43:1 (1999), pp. 1-18.

FORMULA

Jean Lagrange gave a parametric representric representation to produce 6-tuples (a, b, c, d, e and f) such that sum of any two of the numbers is a square. It depends on two parameters x and y where he specializes the case where y = 1.

a = (80y^6 + 320y^2)x^2 + (64y^4 + 128y^2)x + 32y^2 ==> 1

b = (80y^6 + 320y^2)x^2 - (64y^4 + 128y^2)x + 32y^2 ==> 2

c = (16y^8 - 16y^6 + 32y^4 - 64y^2 - 128)x^2 - 32y^2 + 64  ==> 3

d = -(48y^6 + 192y^2)x^2 - (64y^4 - 128y^2)x + 32y^2 ==> 4

e = -(48y^6 + 192y^2)x^2 + (64y^4 - 128y^2)x + 32y^2 ==> 5

f = -(8y^8 + 16y^6 - 32y^4 + 64y^2 - 256)x^2 + 16m^4 - 32m^2 ==> 6

But, for any rational number y there are only finite values of x such that a, b, c, d, e and f in 1, 2, 3, 4, 5 and 6 are distinct integers which form a set with the property that sum of any two numbers in the set is a perfect square.

The formula used to generate triples (k, l, m) such that sum of any two numbers is a square where a, b and c are any parameters are

k = (b^2 + c^2 - a^2)/2

l = (c^2 + a^2 - b^2)/2

m = (a^2 + b^2 - c^2)/2

The formula used to generate quadruplets (k, l, m, n) such that sum of any two numbers is a square is where s is any number that is the sum of two distinct squares in three ways and a, b and c are any three parameters is

k = (b^2 + c^2 - a^2)/2

l = (c^2 + a^2 - b^2)/2

m = (a^2 + b^2 - c^2)/2

n = s - (a^2 + b^2 - c^2)/2

The formula to generate 5-tuplets such that sum of any two numbers is a perfect square uses complex trigonometric formulas.

EXAMPLE

In the set of numbers {-15863902, 17798783, 21126338, 49064546, 82221218, 447422978}, if we add any two numbers, all the 15 combinations give a perfect square and 447422978 is the largest number. For example, 21126338 + -15863902 and 49064546 + 82221218 = 11458^2 = 2294^2. Hence a(6) = 447422978. Similarly

{-4878, 4978, 6903, 12978, 31122} is the set of 5 elements, with the smallest lesser element, such that all the 10 combinations of adding any two numbers gives a perfect square. 31122 is the largest number in this set. Hence a(5) = 31122.

{-94, 95, 130, 194} is the set of 4 elements, with the smallest lesser element, such that all six combinations of adding any two numbers is a perfect square. 194 is the largest number in this set. Hence a(4) = 194.

{-4, 4, 5} is the smallest set of 3 elements such that all three combinations of adding any two numbers gives a perfect square.

{-1, 1} is the trivial set since -1 + 1 = 0^2.

CROSSREFS

Cf. A000290, A195898, A195899.

Sequence in context: A049662 A264176 A178018 * A153227 A116159 A054205

Adjacent sequences:  A195851 A195852 A195853 * A195855 A195856 A195857

KEYWORD

nonn

AUTHOR

Kausthub Gudipati, Sep 24 2011

STATUS

approved

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Last modified June 20 04:43 EDT 2021. Contains 345157 sequences. (Running on oeis4.)