Tensor of a quaternion: Difference between revisions

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{{Expert|date=March 2009}}

In [[mathematics]], some thinkers believe there is a relationship between the norm of a [[Classical Hamiltonian quaternions|quaternion]] and the [[tensor]] of a quaternion. Some writers define the norm of a quaternion as having the same formula as the tensor of a quaternion, while other writers define the norm of a quaternion as the square of the tensor. Hamilton uses the term tensor in two different sences as a [[Classical_Hamiltonian_quaternions#Tensor|positive numerical quantity]] and as an operator that operates on other mathematical entities extracting a tensor quantity from them.

Hamilton did not, as now claimed, ''define'' a tensor to be "a signless number"; what he actually says is:

*''it seems convenient to enlarge by definition the signification of the new word tensor, so as to render it capable of including also those other cases in which we operate on a line by diminishing instead of increasing its length ; and generally by altering that length in any definite ratio. We shall thus (as was hinted at the end of the article in question) have fractional and even incommensurable tensors, which will simply be numerical multipliers, and will all be positive or (to speak more properly) SignLess Numbers, that is, unclothed with the algebraical signs of positive and negative ; because, in the operation here considered, we abstract from the directions (as well as from the situations) of the lines which are compared or operated on.''

Hamilton defined the new word ''tensor'' as a positive or more properly signless number.{{Failed verification|date=March 2009}} <ref>[http://books.google.com/books?id=TCwPAAAAIAAJ&printsec=frontcover&dq=tensor+definition+positive+properly+new+word#PRA1-PA57,M1 Hamilton 1853 pg 57 ]</ref><ref>[http://books.google.com/books?id=YNE2AAAAMAAJ&printsec=frontcover&dq=positive+tensor+strictly+speaking+number+without+sign&as_brr=1#PPA5,M1 Hardy 1881 pg 5]</ref><ref>[http://books.google.com/books?id=CGZLAAAAMAAJ&pg=PA146&dq=Hamilton+positive+signless+quotients#PPA31,M1 Tait 1890 pg.31 explains Hamilton's older definition of a tensor as a positive number]</ref>The [[tensor of a quaternion]] is a number which represents its magnitude,<ref>Tait</ref> the "stretching factor"<ref>[http://books.google.com/books?id=CGZLAAAAMAAJ&pg=PA146&dq=Hamilton+positive+signless#PPA31,M1 Tait (1890), pg 32]</ref>, the amount by which the application of the quaternion lengthens a quantity; specifically, the tensor is defined{{Fact|date=March 2009}} as the square root of the [[#Common norm|norm]] <ref>[http://books.google.com/books?id=CGZLAAAAMAAJ&pg=PA146&dq=tensor+called+norm#PPA146,M1 Cayley (1890), pg 146], </ref> — this is a one-dimensional quantity, quite distinct from the modern sense of [[tensor]], coined by [[Woldemar Voigt]] in 1898 to express the work of [[Bernhard Riemann|Riemann]] and [[Gregorio Ricci-Curbastro|Ricci]].<ref>''OED'', "Tensor", def. 2b, and citations.</ref> As a square root, tensors cannot be negative{{Fact|date=March 2009}}, and the only quaternion to have a zero tensor is the zero quaternion{{Fact|date=March 2009}}. Since tensors are numbers, they can be added, multiplied, and divided. The tensor of the product of two quaternions is the product of their tensors; the tensor of a quotient (of non-zero quaternions) is the quotient of their tensors; but the tensor of the sum of two quaternions ranges between the sum of their tensors (for parallel quaternions) and the difference (for anti-parallel ones) .

The tensor of the quaternion ''q'' is denoted '''''T'''q''.

==References==

{{reflist}}

[[Category:Tensors]]

[[Category:Quaternions]]

[[Category:Mathematics articles needing expert attention]]

[[Category:Mathematics articles needing attention]]