A221279 Smallest tau-value of a special set (in the sense of Heath-Brown) with n elements.

1, 2, 12, 120, 5040, 131040

Offset

2,2

Comments

A special set is a subset of the positive integers such that the absolute difference of any two distinct terms is the same as their greatest common divisor. Tau of a set of positive integers is defined as the least common multiple of the absolute differences in the set.

a(n) exists for each n, a consequence of Heath-Brown's 1984 result that there are special sets of arbitrarily large cardinality.

References

D. R. Heath-Brown, Consecutive almost-primes, J. Indian Math. Soc. (N.S.) 52 (1987), pp. 39-49 (1988).

D. R. Heath-Brown, A note on the paper: "Consecutive almost-primes", J. Indian Math. Soc. (N.S.) 66 no. 1-4 (1999), pp. 203-205.

Adolf J. Hildebrand, Multiplicative properties of consecutive integers; pp. 103-118 in Analytic number theory, ed. by Y. Motohashi.

Links

Table of n, a(n) for n=2..7.

D. R. Heath-Brown, The divisor function at consecutive integers, Mathematika 31 (1984), pp. 141-149.

Adolf Hildebrand, On a conjecture of Balog, Proceedings of the American Mathematical Society 95:4 (1985), pp. 517-523.

Formula

Heath-Brown (1988) proved that n log n << log a(n) << n^3 log n.

Example

{1,2} is special since 2-1 = gcd(2, 1). tau({1,2}) = lcm({2-1}) = 1, so a(2) = 1.
{2,3,4} is special since 3-2 = gcd(3,2), 4-3 = gcd(4,3), and 4-2 = gcd(4,2). tau({2,3,4}) = lcm({3-2,4-3,4-2}) = 2, so a(3) = 2.
tau({1,2}) = 1.
tau({2,3,4}) = 2.
tau({8,9,10,12}) = 12.
tau({40,45,48,50,60}) = 120.
tau({210, 216, 220, 224, 225, 240}) = 5040.
tau({49920, 49950, 49952, 49959, 49960, 49968, 49980}) = 131040.

Crossrefs

Sequence in context: A007132 A138534 A062080 * A165300 A028359 A034524

Adjacent sequences: A221276 A221277 A221278 * A221280 A221281 A221282

Keyword

nonn,hard,more

Author

Charles R Greathouse IV, Jan 10 2013

Extensions

a(6) from Charles R Greathouse IV, Jan 20 2013

a(7) from Charles R Greathouse IV, Feb 07 2013

Status

approved