A273594 Least number k such that abs(A011541(n+1) - A011541(n)*k^3) is a minimum.

9, 37, 43, 19, 79

Offset

1,1

Comments

If b is the sum of two positive cubes in exactly n ways, then b*c^3 is the sum of two positive cubes in at least n ways for all c > 0. So A011541(i)*c^3 is a candidate for unknown Taxi-cab numbers. a(6) = 79 is an interesting example that is related to this case. Additionally, the benefit of this simple fact is the determination of upper bounds for unknown Taxi-cab numbers in relatively easy way.

The inequalities that are given in the comment section of A011541 are:

A011541(7) <= 101^3*A011541(6),

A011541(8) <= 127^3*A011541(7),

A011541(9) <= 139^3*A011541(8).

So a(6) <= 101, a(7) <= 127, a(8) <= 139.

Links

Table of n, a(n) for n=1..5.

Example

a(5) = 79 because abs(A011541(6) - A011541(5)*79^3) = abs(24153319581254312065344 - 48988659276962496*79^3) = 0

Crossrefs

Cf. A011541.

Sequence in context: A274769 A126914 A337238 * A041150 A042021 A137184

Adjacent sequences: A273591 A273592 A273593 * A273595 A273596 A273597

Keyword

nonn,hard,more

Author

Altug Alkan, May 26 2016

Status

approved