We define f1¨ by f1¨ (a) ¨ a = f1(a) where ¨ is a multivector product.
Thus, for example, if tensor fp(a) is scalar valued then
¦¿(a)
is the 1-tensor f(a) = (f(a))a (a-2)
satisfying f(a)¿a = f(a) .
There is a strong temptation to abbreviate ¦(a) as ¦a , "ommiting the brackets",
but this is dangerous because ¦a more properly denotes the composite operator
(¦(a))(b) = ¦(ab) .
Confusion can arise in particular in the case of an operator like Ñp which we also
consider to be "like" a 1-vector geometrically. f(Ñ) and fÑ are then
fundamentally different constructs.
We will insert a composition product symbol ° between two operators only when we wish to emphasise an
"assumed" compositional product. The composition product symbol will normally be ommitted
for brevity. Thus
f°g(a) º f(g(a)) º (fg)(a) º fg(a)
How are we to interpret
f1Ñg1 º (f1Ñ)g1 ? The "differentialiser" Ñ converts
f1 from an operator or tensor taking one nonprimary argument to one taking two, it introduces a second
non-primary 1-vector parameter. In nonabbreviated expressions we will add this parameter
at the rightmost end of the parameter list with
f1Ñ(a,b)
= (Ðbf1)(a)
= Ðb(f1(a)) - f1(Ðba) .
We will interpret the g1 in the "composite product" f1Ñg1 as applying to the
"newest" rightmost nonprimary parameter so that
f1Ñg1(a,b)
º
f1Ñ(a,g1(b)) .
From a programmers' perspective, we can think of "parsing" our composite operator expressions from
left to right pushing introduced parameters onto a stack. When a "compositional irregularity" such as
f1Ñg1 is encoutered it is the most "recent" parameter to which they apply in accordance wth
a stanard "last in first out" stack. We will accordingly refer to
f1Ñg1(a,b) º f1Ñ(a,g1(b)) as the LIFO convention.
Tangential 1-differential Ñ
We can define the tangential 1-differential of field Fp
by
FÑ[CM]pp(a)
º
FpÑ(a)
º (a*Ñ[CM]p)F(p)
= (a*¯(Ñp))F(p)
= (¯(a)*Ñp)F(p)
= FpÑ(¯(a))
º Fpѯ(a)
which gives us
Ñ = ѯ
abbreviating
FpÑ(a) =
Fpѯ(a)
º
FpÑ(¯(a)) .
; the a-directed tangential derivative
is equivalent to the ¯(a)-directed derivative.
Consider now (ÑFp)Ñ(a) = ¯Ñ(Ñb,a)FpÑ(b) + ÑbFpÑ2(b,a) [ HS 4-1.18 ]
More generally suppose Fp(a1,a2,...ak) is an extensor tensor of k multivector variables
themselves p-dependant. We can take a 1-differential of the secondary differntial
Ña1Fp(a1,a2,...ak) to give
(Ña1Fp)Ñ(a1,..,ak,b) =
¯Ñ(Ña1,b)Fp(a1,..,ak)
+ Ña1FpÑ(a1,..,ak,b) .
For 1-vector a1 = ¯a1 we can write this as
(b*Ñ)ÑÞ = ¯Ñ(ÑÞ, b) + ÑÞ(b*Ñ)
which we will here refer to as the primary-secondary Ñ commutation rule
[ HS 4-1.19b ] .
Analagous to the gradifying substitution rule we have the tangential gradifying substituion rule
that applying the operatior (Ñß¿ÑÞ) is equivalent to replacing the first nonprimary argument
of a tensor with ÑÜ .
[ Proof : (Ña¿Ñß)Fp(a,b,..)
= åi=1M ei2 Ðaei(ÐpeiFp)(a,b,...)
= åi=1M (ÐpeiFp)(ei2ei,b,...)
= åi=1M (ÐpeiFp)(ei,b,...)
= åi=1M FpÑ(Ðpeiei,b,...)
= FpÑ(Ñ,b,...)
.]
Hypercurve
For M=N-1 we have Ñ = ¯(Ñ) = Ñ - np(np¿Ñ) = Ñ - npÐnp .
Since np¿Ðanp = 0 " a , Ñpnp lies within iN-1p and
^[Ñpnp] = np-1ÙÐnpnp .
More generally, let np be a unit 1-field.
Ñp =
np2Ñp =
np(np¿Ñp)
+ np(npÙÑp)
corresponding to tangential and orthogonal components of
Ñp .
Directed Coderivative Ð()
With regard to a multivector field Fp defined over CM (but not necessarily confined within Ip) we can most simply define the
a-directed coderivative for aÎUN as the projection of the a-directed tangential derivative, ie. the projection
of the ¯Ip(a)-directed dirivative
Ð = ¯ Я() º ¯° Я()
abbreviating ÐaFp º ¯IpЯIpaFp
The symbol Ð can thus be thought of as an abbreviation
Ð = ¯ Я() = Я()¯ - ¯Ñ
abbreviating
Ðb(ap) = ¯( Я(b)ap) = Я(b)(¯(ap)) - ¯Ñ(ap,b) [ HS 5-1.1 ]
. The underscore serves to remind us of a p and Ip (ie. CM) dependance.
[ Proof : Я(d)(¯(ap)) = ( Я(d)¯)(ap) + ¯( Я(ap)) Þ
Ðd(ap) º ¯( Я(d)(ap)) = Я(d)(¯(ap)) - ( Я(d)¯)(ap)
= Я(d)(¯(ap)) - ¯Ñ(ap)
.]
The directed coderivative of an extended field is defined by
(ÐdFp)(a1,a2,...)
º Ðd(Fp(a1,a2,..)) - Fp(Ðda1,a2,...)- Fp(a1,Ðda2,...)
= ¯( Я(d)(Fp(a1,a2,..)) ) - Fp(¯( Я(d)a1),a2,...)- Fp(a1,¯( Я(d)a2),...)
Note that this differs from [ HS 4-3.3 ] which inserts subtracts
- Fp(Ðd¯(a1),a2,...)- Fp(a1,¯( Я(d)a2),...)
Coderivative Ñ
The covariant derivative Ða within an M-curve can be approached in
a number of creative ways. One is to simply write down all the properties we would like a derivative to have
( such as ÑpFp Î Ip ; Ðpa1p + Ðpa2p = Ðp(a1p+a2p) ; and so forth )
and then intone "as defined so mote it be" three times at midnight. Another is based on projections
[see General Relativity ]. Many appeal to a notion of
"parallel transport" which is defined (or not) in a variety of ways.
We define the undirected coderivative 1-vector operator as the projected tangential derivative
ÑFp º Ñ[Ip]pFp º ¯Ip(Ñ[Ip]pFp)
which we can express operationally as Ñ º ¯Ñ
noting carefully that this denotes operator composition
¯°Ñ º ¯(Ñ(ap)) rather than
¯(Ñ)(ap)
= ¯(¯(Ñ))(ap)
= ¯(Ñ)(ap)
= Ñ(ap)
. The differentiating scope of the Ñ is to be thought of as extending rightwards only
in the usual manner, and not effecting the ¯ .
We have the operator identity Ða = (a¿Ñp) .
Since
ÑFp Î Ip for a scalar field Fp
the coderivative operator Ñp is equivalent to the tangential
derivative Ñp when acting on scalar fields, and for aÎIp the a-directed coderivative
Ða is equivalent to the a-directed derivative Ða when acting on scalar fields.
Ñp(ap) º ¯(Ñp(ap)) = Ñp(¯(apÑ))
by the projected product rule .
This is particularly clear when expressed in coordinate terms with a fortuitous basis as
Ñp(¯(apÑ))
= åi=1N ei Я(ei)(¯(apÑ))
= åi=1M ei(¯( Я(ei)ap))
= åi=1M ¯(ei)(¯( Я(ei)ap))
= åi=1M ¯(ei( Я(ei)ap))
= ¯(ei(åi=1M Я(ei)ap))
= ¯(Ñp(Ap))
º Ñp(Ap)
.
For an extended field Fp(a1p,a2p,...akp) with aipÎUN we have
a-directed primary coderivative
Ð↓aFp(a1p,a2p,...akp) º
(ÐaFp)(a1p,a2p,...akp)
= Ða(Fp(a1p,a2p,...akp))
- Fp(Ða(¯Ip(a1p)),a2p,...akp)
- Fp(aip,Ða(¯Ip(a2p)),,...akp)
- Fp(aip,a2p,...,Ða(¯Ip(akp)))
and an ith exterior coderivative
Ñi→¿p Fp(a1p,a2p,...akp)
º
¯Ip(Ñi→¿p Fp(a1p,a2p,...akp))
, the projection of the ith conveyed divergence.
The cogradifying substitution rule that applying the operator (Ñß¿ÑÞ) = (Ñß¿_csgrad) º ¯°(Ñß¿ÑÞ) is equivalent to replacing the first nonprimary parameter with ÑÜ defined by follows immediately as the projection of tne tangential gradifying substitution rule.
We will see that ÑÙÑÙap = 0 " path-independant ap , so that Ñ2 , while not a scalar operator, does not increase grade .
Hypercurve
For M=N-1 we have Ñnp = Ñnp .
Projection Differential (1.2)-tensor ¯Ñ
¯Ñ(ap,d) º ¯IpÑp(ap,d) º
Ðd(¯Ip(ap)) - ¯Ip(Ðdap)
(informally)
the rate of change of ¯Ip(a) in direction d , is of
of less interest than ¯Ñ, the primary tangential 1-differential of ¯Ip
(ie. the second tangential 1-differential of the identity function) ,
¯Ñp(ap,d) =
¯Ñp(ap,¯d) =
1Ñ2(ap,d)
= ( Я(d)¯Ip)(ap)
= Я(d)(¯(ap)) - ¯( Я(d)ap)
= 2( Я(d)ׯ)(ap) .
The symmetry
¯Ñ(a,b)
=¯Ñ(b,a)
for a,b Î Ip follows from the symmetry of 1Ñ2 in UN.
Tangentially differentiating the outtermorphism result ¯Ip(aÙb)=¯Ip(a) Ù ¯Ip(b) yields
¯Ñ(aÙb,d)
= ¯Ñ(a,d) Ù ¯(b)
+ ¯(a) Ù ¯Ñ(b,d)
[ HS 4-2.6 ] and hence
¯Ñ(a1Ùa2Ù....ak,d)
= åi=1k (-1)i+1
¯Ñ(ai,d)Ù¯Ip(a1)Ù...
¯Ip(ai-1)Ù
¯Ip(ai+1)Ù...
¯Ip(ak) .
[ HS 4-2.31 ]
¯¯Ñ = ¯Ñ^ ; ^¯Ñ¯ = ¯Ñ¯
[ HS 4-2.11 ]
and so
¯=¯Ñ = ^=¯Ñ = 0
abbreviating ¯(¯Ñ(¯(a),b)) = ^(¯Ñ(^(a),b)) = 0 " a,b.
[ Proof : Tangentially differentiating ¯2(a) =¯(a)
gives
¯Ñ(a,d)
= ¯Ñ(¯(a),d) + ¯(¯Ñ(a,d))
Þ
¯¯Ñ(a,d) = ¯Ñ((1-¯)(a),d) =
¯Ñ(^(a),d) .
Thus ¯=¯Ñ = ¯Ñ ^¯ = ¯Ñ 0 = 0
.]
Trivially therefore, ¯¯Ñ(Ip,d) = 0 and hence ¯Ñ(Ip,d)¿Ip = 0
" d .
ÑÞ¿(¯Ñ¯) = 0 abbreviating
Ñb¿¯Ñ(¯a,b) = 0 " a,b [ HS 4-2.17a ] .
[ Proof : ÑÞ¿¯Ñ(¯a,b) (¯(ÑÞ))¿¯Ñb)
= ÑÞ¿(¯¯Ñb)) = ÑÞ¿0 .
.]
ÑÞÙ(¯Ñ^) = 0
[ Proof :
See [ HS 4-2.17b ]
.]
¯Ñ enables us to express the tangential differential of the undirected derivative
in terms of the first and second tangential differential as
(ÑFp)Ñ(a) = ¯Ñ(Ña,b)FpÑ(a))
+ ÑaFpÑ2(a,b)
provided aÎIp .
[ Proof :
aÎIp Þ FpÑ(a)
= (a¿Ñ)Fp
= (¯(a)¿Ñ)Fp
= (¯(a)¿Ñ)Fp
= FpÑ(¯(a))
.]
Hypercurve
For M=N-1 we have ¯Ñ(ap,b) = (Ðb¯)(ap)
= -np2((ap¿npÑ(b))np + (ap¿np)npÑ(b)) .
Thus ¯Ñ(¯(ap),b)
= -np2 (¯(ap)¿npÑ(b))np
= -np2 (ap¿npÑ(b))np ;
Thus ¯Ñ(np,b) = -npÑ(b) .
, the normalisation condition on np providing np¿npÑ(b)=0 .
Projection Second Differential (1;3)-tensor ¯Ñ2
We can take the second tangential differential of ¯, ie. the third tangential differntial of 1
obtaining
¯Ñ2(a,b,c) º
Я(c)(¯Ñ(a,b)) - ¯Ñ( Я(c)a,b)) - ¯Ñ(a, Я(c)b)) .
Squared Projection Differential (1;3)-tensor (¯Ñ)2
(¯Ñ)2(a,b,c) º ¯Ñ(¯Ñ(a,b),c)
(¯Ñ)2ׯ = Рׯ
abbreviating
(¯Ñ)2(¯Fp,b,a) - (¯Ñ)2(¯Fp,a,b) =
Ða(Ðb(¯Fp))-Ðb(Ða(¯Fp))
[ Proof : Recalling ¯=¯Ñ=0 we have:
ÐaÐb(¯Fp)
= Ða( Я(b)(¯Fp) - ¯Ñ(¯Fp,b) )
= ¯( Я(a)( Я(b)(¯Fp) - ¯Ñ(¯Fp,b) ))
= ¯=( Я(a) Я(b))(Fp) - ¯( Я(a)(¯Ñ(¯Fp,b)) )
= ¯=( Я(a) Я(b))(Fp)
- ¯[ ( Я(a)¯Ñ)(¯Fp,b)
+ ¯Ñ( Я(a)(¯Fp),b)
+ ¯Ñ(¯Fp, Я(a)b) ]
= ¯=( Я(a) Я(b))(Fp)
- ¯¯Ñ2(¯Fp,b,a)
- ¯¯Ñ( Я(a)(¯Fp),b)
= ¯=( Я(a) Я(b))(Fp)
- ¯=(¯Ñ2)(Fp,b,a)
- ¯¯Ñ( ( Я(a)¯)Fp)+¯( Я(a)Fp),b)
= ¯=( Я(a) Я(b))(Fp)
- ¯=(¯Ñ2)(Fp,b,a)
- ¯(¯Ñ)2(Fp,a,b)
Þ (Ða×Ðb)Fp
=
(¯Ñ)2×(Fp,a,b)
by symmetry of ¯Ñ2 and integrability condition Я(a)× Ð¯(b)=0
.]
Shape (<0.2>;1)-multitensor [ѯ]
We define the shape
[ѯ]
of CM to be the undirected tangential 1-derivative
of the projector , an abbreviation of
[ѯ](ap) º Ñß[CM]p¯Ip(ap)
º Ñß ¯Ip(ap)
= (Ñb Ðp↓(b) ¯Ip)(ap)
= Ñb ¯Ñ(ap,b)
= Ñb ¯Ñ(ap,b)
= Ñb ¯Ñ(ap,b)
.
[ѯ](ap) = 0 so [ѯ] annihilates scalars.
[ѯ](ap) decomposes as [ѯ](^Ip(ap)) + [ѯ](¯Ip(ap))
and, joy of joys, this corresponds precisely to its grade decomposition into scalar and bivector parts.
We can express this immense good fortune operationally
with regard to general operands as
[ѯ] ^ = ¯ [ѯ] = [Ñ¿¯]
[ HS 4-2.35b ] which stems from
[ÑÙ¯]^ = [Ñ¿¯]¯ = 0 holding generally [ HS 4-2.37 ] ; and
[ѯ]¯ = ^[ѯ] = [ÑÙ¯] [ HS 4-2.35a ]
since ^[ѯ] = [ѯ]-¯[ѯ] = [ѯ]-[ѯ]^ = [ѯ]¯ .
For general operands we have
[ѯ](a)<k+1> = [ѯ](¯Ip(a<k>)) ;
[ѯ](a)<k-1> = ¯Ip([ѯ](a<k>))
and so
ѯ = ѯ + [ѯ]¯ = ѯ + [ÑÙ¯]
[ HS 4-3.7a ] with
Ñ¿¯ = Ñ¿¯ . Hence
^ѯ = [ѯ]¯ = [ÑÙ¯]¯
[ Proof : Ñ(¯(ap)) = Ñ(¯(ap)) + ^Ñ(¯(ap))
= ѯ(ap) + ^( [ѯ](ap) + ѯ(apÑ) )
= ѯ(ap) + ^([ѯ](ap) + Ñ(ap))
= ѯ(ap) + ^[ѯ](ap)
= ѯ(ap) + [ѯ]¯(ap)
= (Ñ+[ѯ])¯(ap) .
.]
With tangential operands ¯(ap)=ap understood we can write this as
Ñ = Ñ + [ѯ] = Ñ + [ÑÙ¯] [ HS 4-3.6a ]
and ^Ñ = [ѯ] = [ÑÙ¯] .
[ÑÙ¯](a) º ([ѯ](a))<2> = [ѯ]¯(a) is known as the curl (2;1)-tensor .
Scalar ¯1 [ѯ](a) =[ѯ](^Ip(a)) = (Ñp¿¯Ip)(a)
is expressible
as (Ñb[ÑÙ¯](b))¿a where
1-vector
Ñb[ÑÙ¯](b) =
º ÑÞ[ÑÙ¯](b) =
ÑÞ¿[ÑÙ¯](b)
º ÑÞ¿[ÑÙ¯](b)
[¯(Ñ)]
= ¯Ñ(Ñ) = ÑÞ [ÑÙ¯] = ÑÞ¿[ÑÙ¯]
= [Ñ¿¯]¿
[ HS 4-2.20 ]
is known as the spur 1-field of CM and lies outside Ip .
[Ñ¿¯](a) = [¯(Ñ)].a [ HS 4-2.18 ] so the shape (<0,2>;1)-tensor is "recoverable" from the curl (2;1)-tensor as [ѯ](a) = [ÑÙ¯](a) + (Ñb[ÑÙ¯](b)).a .
1-vector ¯Ip(b)¿[ѯ](a) = ¯Ip(b)¿[ÑÙ¯](a) is normal to CM , ie. ¯Ip ( ¯Ip(b)¿[ѯ](a)) = 0 .
[ѯ](a1Ùa2Ù...ak) = åi=1k. (-1)i+1 [ [ѯ](ai) Ù ¯Ip(a1Ù..Ùai-1Ùai+1..Ùak) + ¯Ip([ѯ](ai) × (a1Ù..Ùai-1Ùai+1..Ùak) ] [ HS 2.41c ] provides the extension of [ѯ] to general multivectors.
[ѯ] = ÑÞ2 ¯Ñ
abbreviating [ѯ](ap) = Ñb¯Ñb)
expressing the shape as the secondary tangential derivative of ¯Ñ
[ HS 4-2.14 ].
[ Proof : Ðbd ¯Ñ(ap,b)
= Ðbd( Я(b)(¯(ap)) - ¯( Я(b)ap))
= Lime ® 0 e-1[ Я(b+ed)(¯(ap)) - ¯( Я(b+ed)ap))
- Я(b)(¯(ap)) - ¯( Я(b)ap))]
= Я(d)(¯(ap)) - ¯( Я(d)ap))
= ( Я(d)¯)(ap)
so we have both
Ñß ¯(ap)
º (ѯ)(ap) = Ñb¯Ñ(ap,b) º ÑÞ2¯Ñ(ap,b)
and
Ñ¯ß ¯(ap)
º [ѯ](ap) = Ñb¯Ñ(ap,b) º ÑÞ2¯Ñ(ap,b)
Since ¯Ñ(a,b)=¯Ñ(b,a) the result follows.
.]
The grade-tangency associations of [ѯ] immediately provide
[ѯ]^ =
[Ñ¿¯] = ÑÞ2¿¯Ñ
;
[ѯ]¯ =
[ÑÙ¯] = ÑÞ2Ù¯Ñ
[ HS 4-2.16 ].
[ Proof : [ѯ](a) º (Ñp¯))(a) = Ñb¯Ñp(a,b) so .... ???
.]
Ñ2 = [ѯ](Ñ) + ¯Ñ2
[ Proof : Ñ2(ap) =Ñ(¯Ñ(ap)) = [ѯ](Ñ) + ¯Ñ(Ñ(ap))
= [ѯ](Ñ)(ap) + ¯Ñ(Ñ(ap))
= [ѯ](Ñ)(ap) + ¯ÑÑ(ap)
= [ѯ](Ñ)(ap) + ¯¯ÑÑ(ap)
= [ѯ](Ñ)(ap) + ¯Ñ2(ap)
= [ѯ](Ñ)(ap) + ¯(Ñ¿Ñ)ap
.]
Hence ÑÙÑ = [ѯ](Ñ) = [ÑÙ¯](Ñ) with regard to path-independant functions.
[ HS 4-3.10b ] .
We have the following Shape Properties:
[ѯ] = ÑÞ ¯Ñ enables us to grade extend [ѯ] from the grade extension of_prl0g()
as
[ѯ](aÙb) = Ñc¯Ñ(aÙb,c)
= Ñc(¯Ñ(a,c)Ù¯(b)+¯(a)Ù¯Ñ(b,c))
so we immediately have [ѯ](^(a)Ù^(b))=0 [ HS 4-2.40c ]. Furthermore
[ѯ](¯(aj)Ù¯(b))
= ([ѯ](¯(aj))Ù¯(b) + (-1)j¯(aj)Ù([ѯ]¯(b))
[ HS 4-2.40a ]
[ Proof :
[ѯ](¯(aj)Ù¯(bk))
= [ѯ]¯(ajÙbk)
= [ÑÙ¯](ajÙbk)
= ÑcÙ( ¯Ñ(¯(aj),c)Ù¯(bk) + (¯(aj)Ù¯Ñ(¯(bk),c) )
= ÑcÙ( ¯Ñ(¯(aj),c)Ù¯(bk) + (-1)jk¯Ñ(¯(bk),c)Ù¯(aj) )
= (ÑcÙ¯Ñ(¯(aj),c))Ù¯(bk)
+ (-1)_jk(ÑcÙ¯Ñ(¯(b),c))Ù¯(aj)
= ([ÑÙ¯](¯(aj))Ù¯(bk) + (-1)jk([ÑÙ¯]¯(b))Ù¯(aj)
= ([ÑÙ¯](¯(aj))Ù¯(bk) + (-1)jk+j(k+1)¯(aj)Ù[ÑÙ¯]¯(bk)
= ([ѯ](¯(aj))Ù¯(bk) + (-1)j¯(aj)Ù([ѯ]¯(bk))
.]
In particular, [ѯ]¯(aÙb) = [ѯ]¯(a)Ù¯(b)-[ѯ]¯(b)Ù¯(a) .
[ѯ](^(aj)Ùb) =
[ѯ](^(aj)ٯ(b)) =
[ѯ](^(aj))ٯ(b)
+ (-1)j¯Ñ(^(aj))Ù(Ñ¿b) [ HS 4-2.40b ]
[ Proof :
[ѯ](^(aj)ٯ(bk))
= Ñc¯Ñ(^(aj)Ù¯(bk) ,c)
= Ñc(¯Ñ(^(aj),c)Ù¯(bk))
= Ñc¿(¯Ñ(^(aj),c)Ù¯(bk))
+ (ÑcÙ¯Ñ(^(aj),c))Ù¯(bk)
= (Ñc.¯Ñ(^(aj),c))Ù¯(bk) + (-1)j ¯Ñ(^(aj),c)(Ñc.¯(bk))
+ [ÑÙ¯](^(aj))Ù¯(bk) by the expanded inner product rule
= ([ѯ]^(aj) + (-1)j ¯Ñ(^(aj),c)(Ñc.¯(bk))
+ 0
.]
====
The primary 1-differential of the extended curl (2;1)-tensor is
[ÑÙ¯]Ñ(a,b) º Я(b) [ÑÙ¯](ap) - [ÑÙ¯]( Я(b)(ap))
.
Hypercurve
For M=N-1 we have [ÑÙ¯](a) = np2 npnpÑ(a) = np2 npÙnpÑ(a)
[ Proof : ÐbiN-1p
= Ðb(npi)
= (Ðbnp)i
= npÑ(b)i so Shape Property 7 gives
[ÑÙ¯](a) =
iN-1p-1( Я(a)iN-1p)
= i-1np-1npÑ(a)i
= np2 _psiinvd(npnpÑ(a))
= np2 _psiinvd(npÙnpÑ(a))
= np2 npÙnpÑ(a) since i commutes with all bivectors.
.]
[¯(Ñ)] = np2 np(Ñp¿np)
= np2 np(Ñp¿np)
[ Proof :
Ña[ÑÙ¯](a) = -np2 Ña[npÑ(a)np]
= -np2 (ÑanpÑ(a))np
= -np2 (ÑaÐanp)np
= -np2 (Ñpnp)np
= -np2 np(Ñp¿np)
.]
Thus [Ñ¿¯](a) = [¯(Ñ)].a = -np2 (Ñ¿np)(ap¿np)
Squape 1-multitensor [ѯ]2
The shape tensor [ѯ] raises the grade of
¯Ip(a) while lowering the grade of ^Ip(a), preserving neither
grade nor tangency (containment within Ip) but
the
squared shape or squape 1-tensor
[ѯ]2 preserves both with
[ѯ]2 = ^=([ѯ]2) + ¯=([ѯ]2) .
[ Proof :
Follows from ¯=[ѯ] = ^=[ѯ] = 0 and [ѯ]^ = ¯[ѯ] since
[ѯ]2 = (¯+^)[ѯ](¯+^)[ѯ](¯+^)
= ^[ѯ]¯[ѯ]^ + ¯[ѯ]^[ѯ]¯
= ^[ѯ]2^2 + ¯[ѯ]2¯
.]
We also have [ѯ]2^ = ^[ѯ]2
= [ѯ]¯[ѯ] º [ѯ]=¯ . [ HS 4-2.46b ]
The squape tensor decomposes as [ѯ]<2> +
[ѯ]<1> + [ѯ]<0>
Ricci 1-tensor ([ѯ]2¯)
The intrinsic squape 1-multitensor
is the projection of the squape
¯[ѯ]2 = [ѯ]2¯ = ¯=([ѯ]2) , acting entirely upon and within Ip .
We refer to the instrinsic squape acting only on 1-vectors as the
Ricci 1-tensor.
We will show eventually that
[ѯ]2 ¯
= (ÑÙÑ) ¯
= (ÑÙÑ) ׯ
[ HS 5-1.28 5-1.29 ]
abbreviating
[ѯ]2(¯(ap))
= (ÑÙÑ)(¯(ap))
= (ÑÙÑ)×(¯(ap)) " ap .
Hence ÑÙÑ is grade-preserving.
[ѯ]2¯(aÙb)
= [ѯ]2(a) Ùb + 2¯( [ѯ]Ñ(b)×[ѯ]Ñ(a) )
+ a٠[ѯ]2(b)
[ HS 4-2.48 ]
[ Proof : ... ???
.]
¯[ѯ]2 = -Ñ[ÑÙ¯] = -Ñ¿[ÑÙ¯]
abbreviating
¯[ѯ]2(b) = -Ñp[ÑÙ¯](b) = -Ñp¿[ÑÙ¯](b) .
[ HS 5-1.19 ]
[ Proof :
Ña ¯[ѯ]×(a,b) =
-ÑaÐa[ÑÙ¯](b)
= -Ñp[ÑÙ¯](b) by Curvature Identity 2 below
.]
¯[ѯ]2 is symmetric (self-adjoint).
[ Proof :
???
.]
Curvature 2-tensor [ѯ]×
Recall that the commutator product of two bivectors is itself a bivector.
If Ip satisfies the integrability condition then
the antisymmetric full curvature 2-tensor
cpaÙb º [ѯ]×(a,b) º
[ѯ](a)×[ѯ](b) = [ÑÙ¯](a)×[ÑÙ¯](b)
satisfies
[ѯ]×(a,b) = ½([ÑÙ¯]Ñ(a,b) - [ÑÙ¯]Ñ(b,a))
[ HS 4-4.17 ] where
[ÑÙ¯]Ñ(ap,b) º Я(b)([ÑÙ¯](ap)) - [ÑÙ¯]( Я(b)ap) .
[ Proof : ????? See HS
.]
We can write this
with regard to 1-vector arguments as
[ѯ]×
= [ÑÙ¯]×
= ([ÑÙ¯]Ñ)× º
[ÑÙ¯]Ñ×
Thus we can categorise some second dervivative properties of an M-curve geometrically from the first derivative shape.
Curvature differential (2;3)-tensor [ѯ]×Ñ
This enables us to express the primary differential of the curvature:
[ѯ]×Ñ(a,b,c) º ([ѯ]×(a,b))Ñ(c)
º Ðßc([ѯ]×(a,b))
º (Ðc([ѯ]×))(a,b)
as
½([ÑÙ¯]Ñ(a,b,c)-[ÑÙ¯]Ñ2(b,a,c)) = ½([ÑÙ¯]Ñ(a,b,c)-[ÑÙ¯]Ñ2(b,c,a)) .
Cyclically permuting the a,b,c and summing we obtain the generalised Bianchi identity
S←abc ([ÑÙ¯](a)×[ÑÙ¯](b))Ñ(c) = 0
S←abc ([ѯ]×)Ñ(a,b,c)
º S←abc Ðßc([ѯ]×)(a,b)
= 0 [ HS 5-1.39 ]
which we can also write as ([ѯ]×)Ñ→¿ = 0 . The curvature
thus has vanishing exterior 1-differential.
Setting a=[ѯ](a) in Projection Property 6 [ [ HS 4-2.33 ]] gives
¯Ñ([ѯ](a),b) = ¯([ѯ](a))×[ѯ](b) - ¯([ѯ](a)×[ѯ](b))
= - ¯([ѯ](a)×[ѯ](b))
º - ¯([ѯ]×(a,b))
which we can abbreviate (with 1-vector arguments understood) to
¯ [ѯ]×
= ¯ [ÑÙ¯]×
= -¯Ñ [ÑÙ¯]
ie. 1Ñ2 [ÑÙ¯] = - 1Ñ [ÑÙ¯]× = - 1Ñ [ѯ]×
.
Since the curvature 2-tensor is skewsymmetric and bilinear in its two nonprimary arquments
it defines a bivector-valued curvature (*1)-multitensor] of a single bivector argument
[ѯ]×(aÙb) º [ѯ]×(a,b) . It is natural to extend this to a multivector argument
via
[ѯ]×(a) = [ѯ]×(aÙ1) = [ѯ]×(a,1) = 0 .
º [ѯ]×(a,b) . It is natural to extend this to a multivector argument
via [ѯ]×(a)=[ѯ]×(a)=0
Intrinsic curvature 2-tensor ¯[ѯ]×
The intrinsic curvature of an M-curve
can be defined in a number of ultimately equivalent ways. We will here regard it primarily
as the projection of the full 2-curvature
¯( [ѯ](a)×[ѯ](b)) = ¯([ÑÙ¯](a)×[ÑÙ¯](b) )
and denote it by
¯[ѯ]× = ¯[ÑÙ¯]×
with
¯[ѯ]×(a,b) º ¯( [ѯ](a)×[ѯ](b) )
.
It is bivector-valued and bilinear in a,b and is thus an antisymmetric 2-tensor, ie. a 2-form.
The remaining, rejected, component of the full curvature existing outside Ip is known as the
extrinsic curvature 2-tensor.
Riemann curvature 2-tensor ¯=[ѯ]×
The Riemann curvature of an M-curve is the intrinsic curvature
restricted to the M-curve
¯=[ѯ]×(a,b) º ¯[ѯ]×(¯(a),¯(b))
= ¯([ѯ](¯a)×[ѯ](¯ |b))
= ¯([ÑÙ¯](a)×[ÑÙ¯](b))
= ¯[ѯ]×(¯(aÙb))
so that nonprimary arguments outside Ip are mapped to 0.
(¯[ѯ]×)Ñ→¿(a,b,c)
= S←abc ¯([ѯ]×^(aÙb))×[ѯ]¯(c)
with the immediate consequence
(¯=[ѯ]×)Ñ→¿(a,b,c) = 0 , ie. the
Riemann curvature has vanishing exterior 1-codifferential, which is the traditional
second Bianchi identity.
[ Proof : Ðßc ¯[ѯ]×(aÙb)
º (Ðc ¯[ѯ]×)(aÙb)
= ¯Ñ , c)) + ¯( Ðßc[ѯ]×(aÙb))
Þ S←abc (Ðc ¯[ѯ]×)(aÙb)
= S←abc ¯([ѯ]×(aÙb))×[ѯ](c) .
Hence
S←abc (Ðc ¯[ѯ]×)(aÙb)
= S←abc ¯Ñ , c) + ¯( S←abc Ðßc[ѯ]×(aÙb))
= S←abc ¯Ñ , c) + ¯( 0)
= S←abc (¯([ѯ]×(aÙb))×[ѯ](c)
- ¯([ѯ]×(aÙb) ×[ѯ](c) ) by Shape Proprety 6
= S←abc ¯([ѯ]×(aÙb))×[ѯ](c)
- ¯(S←abc (([ѯ](a)×[ѯ](b))×[ѯ](c) )
= S←abc ¯([ѯ]×(aÙb))×[ѯ](c) by Jacobi Identity .
.]
Hence
S←abc (Ðc ¯[ѯ]×)(aÙb)
= S←abc ¯( Я(c) ¯[ѯ]×)(aÙb) )
= S←abc ¯( ¯([ѯ]×(aÙb))×[ѯ](¯(c)) )
= S←abc ¯( [ѯ]×(aÙb))×[ѯ](¯(c)) ) by the projected bivector commutation rule
Replacing (aÙb) with ¯(aÙb) gives a vanishing result by the generalised Bianchi identity.
.]
We can express the Riemann curvature ¯( [ѯ](a)×[ѯ](b))
solely
in terms of the intrinsic squape multitensor via
¯[ѯ]×(aÙb) = ½( [ѯ]2(a)Ùb + aÙ[ѯ]2(b) - [ѯ]2(aÙb) )
" a,b Î Ip
[ HS 4-2.48 ].
Since [ѯ]2 preserves grade we must have
Ña¿[ѯ]2(aÙb) = -2Ña¿(¯[ѯ]×(a,b))
= -2¯(Ña)¿([ѯ]×(a,b))
= -2Ña¿([ѯ]×(a,b))
and similarly
Ña¿[ѯ]2(aÙb) = -2Ña¿(¯[ѯ]×(a,b))
= [ѯ]2(b) .
The Riemann curvature is protractionless
ÑaÙ¯[ѯ]×(¯(aÙb)) = 0
[ HS 5-1.11 ] .
We also have
ÑßÙ¯[ѯ]ׯ = 0
[ HS 5-1.13a ]
[ Proof :
See [ HS p191 ]
.]
There are various alternate and equivalent definitions for the intrinsic 2-curvature one can adopt based on the following Curvature Identites.
It is easy to fail to appreciate its true significance of this. In consequence of the integrability condition of Ip, it is possible to evaluate (Ða×Ðb)Fp without having to differentiate Fp by applying a particular linear function independant of Fp to Fp. This is why ÑÙÑ is essentially geometric, with no differentiating component.
Hypercurve
For M=N-1 we have [ѯ]×(a,b) = ¯[ѯ]×(a,b) = - npÑ(aÙb) .
[ Proof : [ѯ](a)×[ѯ](b) = [ÑÙ¯](a)×[ÑÙ¯](b) =
np2 (npnpÑ(a))×(npnpÑ(b))
= -np4 (npÑ(a)×npÑ(b))
= - npÑ(a)ÙnpÑ(b)
º - npÑ(aÙb)
.]
Hence [ѯ]2¯(b) = Ña¿¯[ѯ]×(a,b)
= -Ña¿(npÑ(a)ÙnpÑ(b))
= ???
Scalar Curvature (0;1)-tensor R = ÑÞ [ѯ]2¯
The scalar curvature (aka. total curvature)
is traditionally presented as the second contraction of the Rieman curvature
R º
(ÑÞ¿)2 ¯[ÑÙ¯]×
= ÑÞ2 ¯[ÑÙ¯]×
= ÑÞ [ѯ]2¯ ;
but is also, perhaps more fundamentally,
the tangential point divergence of the spur Ñ¿[¯(Ñ)] [ HS 5-1.21 ] .
Ñp¿[ѯ]2¯(a) = Ñp¿[ѯ]2¯(a) = Ñp¿[ѯ]2¯(a)
2 [ѯ]2Ñ(Ñ) = ÑR
[ HS 5-1.23 ]
[ Proof :
Contracting the contracted Bianchi identity yields
Ñb¿((ÑÙ([ѯ]2¯))(b)) =
Ñb¿(¯=[ѯ]×Ñ(ÑÙb))
Þ
(Ñb¿Ñ)([ѯ]2¯)Ñ(b) -
Ñ(Ñb¿([ѯ]2¯)Ñ(b)) = -
Ñb¿(¯=[ѯ]×Ñ(bÙÑ))
Þ
([ѯ]2¯)Ñ(Ñ)) - Ñ(RÑ) = -([ѯ]2¯)Ñ(Ñ))
Þ
2([ѯ]2¯)Ñ(Ñ)) = Ñ(RÑ) .
.]
Einstein 1-tensor (1-½ÑÞ)[ѯ]2¯
2([ѯ]2¯)Ñ(Ñ)) = Ñ(RÑ) means that the symmetric (self-adjoint)
Einstein 1-tensor
(1-½ÑÞ)[ѯ]2¯(a) = ([ѯ]2¯)(a) - ½Ra
= [ѯ]2¯(a) - (Ñ¿[¯(Ñ)])a
has (1-½ÑÞ)[ѯ]2¯Ñ(Ñ) = ¯(1-½ÑÞ)[ѯ]2¯Ñ(Ñ) = 0 and so is zero (point) codivergent, ie.
[Ñ¿(1-½ÑÞ)[ѯ]2¯](a) = [Ñ¿(1-½ÑÞ)[ѯ]2¯](a) = 0 " a Î Ip
.
[ HS 5-1.23 ]
It's (directional) cocontraction is Ña¿((1-½ÑÞ)[ѯ]2¯(a)) = Ña¿((1-½ÑÞ)[ѯ]2¯(a))
= ½(2-M)R .
(1-½Ña)(ÑpÙÑp) = 0
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