Basic Multivector Calculus, Proceedings of FAN 2008, Hiroshima, Japan, 23+24 October 2008
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E. Hitzer, Basic Multivector Calculus, Proceedings of FAN 2008, Hiroshima, Japan, 23+24 October 2008. Basic Multivector Calculus Eckhard Hitzer, Department of Applied Physics, Faculty of Engineering, University of Fukui Abstract: We begin with introducing the generalization of real, complex, and quaternion numbers to hypercomplex numbers, also known as Clifford numbers, or multivectors of geometric algebra. Multivectors encode everything from vectors, rotations, scaling transformations, improper transformations (reflections, inversions), geometric objects (like lines and spheres), spinors, and tensors, and the like. Multivector calculus allows to define functions mapping multivectors to multivectors, differentiation, integration, function norms, multivector Fourier transformations and wavelet transformations, filtering, windowing, etc. We give a basic introduction into this general mathematical language, which has fascinating applications in physics, engineering, and computer science. more didactic approach for newcomers to geometric algebra. 1. Introduction G(I) is the full geometric algebra over all vectors in the “Now faith is being sure of what we hope for and certain of what we do not see. This is what the ancients were n-dimensional unit pseudoscalar I e1 e2 ... en . commended for. By faith we understand that the universe was formed at God’s command, so that what is seen was not made 1 A G (I) is the n-dimensional vector sub-space of out of what was visible.” [7] n The German 19th century mathematician H. Grassmann had grade-1 elements in G(I) spanned by e1,e2 ,...,en . For the clear vision, that his “extension theory (now developed to geometric calculus) … forms the keystone of the entire 1 vectors a,b,c An G (I) and scalars , ,, ; structure of mathematics.”[6] The algebraic “grammar” of this universal form of calculus is geometric algebra (or Clifford G(I) has the fundamental properties of algebra). That geometric calculus is a truly unifying approach associativity to all of calculus will be demonstrated here by developing some basics of the vector differential calculus part of a(bc) (ab)c, a (b c) (a b) c, (1) geometric calculus. The basic geometric algebra[1,2] necessary for this is commutativity a a, a b b a, (2) compiled in section 2. Then section 3 develops vector distributivity differential calculus with the help of few simple definitions. This approach is generically coordinate free, and fully shows a(b c) ab ac, (b c)a ba ca, (3) both the concrete and abstract geometric and algebraic beauty of the “keystone” of mathematics. The full generalization to linearity (a b) a b (a b), (4) multivector calculus is shown in [2,10], applications in [11]. scalar square (vector length ) a 2. Basic Geometric Algebra 2 2 This section is a basic summary of important relationships in a aa a a a . (5) geometric algebra. This summary mainly serves as a reference section for the vector differential calculus to be developed in The geometric product ab is related to the (scalar) inner the following section. Most of the relationships listed here are to be found in the synopsis of geometric algebra and in product a b and to the (bivector or 2-vector) outer product chapters 1 and 2 of [1], as well as in chapter 1 of [2], together with relevant proofs. Beyond that [1] and [2] follow a much a b by ab a b a b , (6) a(A B) aA aB . (19) with The inner and outer products of homogeneous multivectors 1 (13) A and B are defined ([2], p. 6, (1.21), (1.22)) as a b (ab ba) b a ab a b ab , (7) r s 2 0 (13) A B A B for r,s 0 , (20) 1 r s r s r s a b (ab ba) b a ab a b ab (8) 2 2 A B 0 for r 0 or s 0 , (21) The inner and the outer product are both linear and distributive r s A B A B , (22) a (b c) a b a c , (9) r s r s r s a (b c) a b a c . (10) Ar Ar Ar for scalar . (23) A unit vector aˆ in the direction of a is The inner (and outer) product is again linear and distributive a 2 (A ) B A (B ) (A B ) A B (24) aˆ , with aˆ aˆaˆ 1, a aˆ a . (11) r s r s r s r s a A (B C ) A B A C , (25) The inverse of a vector is r s t r s r t 1 1 a a aˆ . (26) a . (12) (Bs Ct ) Bs Ct a a2 a 2 a The reverse of a multivector is A multivector A can be uniquely decomposed into its ~ n A (1)k (k1) / 2 A . (27) homogeneous grade k parts ( grade k selector): k k k1 A A A A ... A ... A (13) [2] uses a dagger instead of the tilde. 0 1 2 k n Special examples are scalar vector bivector k vector pseudo scalar ~ ~ ~ , a a , (a b ) b a a b , …(28) If A is homogeneous of grade k one often simply writes A A A . (14) The scalar magnitude A of a multivector A is k k Grade selection is invariant under scalar multiplication n 2 ~ ~ A A A A 2 A A , (29) A A . (15) 0 r r k k scalar product r1 The consistent definition of inner and outer products of 2 where the separate term A is in particular due to the 0 vectors a and r-vectors A is r definition of the inner product in [2], p. 6, (1.21). The magnitude allows to define the inverse for simple k-blade 1 r a A aA (aA (1) A a), (16) vectors r r r 1 2 r r ~ 1 A 1 1 1 r , with . (30) a A aA (aA (1) A a) (17) A 2 A A AA 1 r r r 1 2 r r A By linearity the full geometric product of a vector and a Alternative ways to express a A G1(I) are multivector A is then n aA a A a A . (18) I a 0 or Ia I a . (31) This extends to the distributive multiplication with arbitrary The projection of a into A G1(I) is multivectors A, B n n n (a b) (c d) a (b (c d)) P (a) P(a) ak a a a ak a, (32) (42) I k k k 1 k 1 (a d)(b c) (a c)(b d ), where ak is the reciprocal frame defined by (a b)2 (a b) (a b) (a b)2 a2b 2 2 if (b a) (a b) a b , (43) k k 1 j k a a j j Kronecker delta . (33) 0 if j k and the Jacobi identity A general convention is that inner products a b and a (b c) b (c a) c (a b) 0. (44) The commutator product of multivectors A,B is outer products a b have priority over geometric products 1 A B (AB BA). (45) ab , e.g. 2 One useful identity using it is a bc de (a b)(c d )e . (34) (a b) A ab A A ab ab A A ab.(46) The The projection of a multivector B on a subspace described by a commutator product is to be distinguished from the cross simple m-vector (m-blade) A a a ... a ,m n m 1 2 m product, which is strictly limited to the three-dimensional is Euclidean case with unit pseudoscalar I : sm 3 P (B) (B A) A1 A1 (A B ), (35) A (s) general a b (b a)I (a b)I . (47) degree dependent 3 3 For more on basic geometric algebra I refer to [1,2,9] and to P ( B ) B , P ( B ) B AA1, (36) A 0 0 A n n section 3 of [3]. the exceptions for scalars B and pseudoscalars 0 3. Vector Differential Calculus This section shows how to differentiate functions on linear B being again due to the definition of the inner product in n subspaces of the universal geometric algebra G by vectors. [2], p. 6, (1.21). A projection of one factor of an inner product has the effect It has wide applications particularly to mechanics and physics in general [1]. Separate concepts of gradient, divergence and a P(b) P(a) P(b) P(a) b. (37) curl merge into a single concept of vector derivative, united by the geometric product. For a multivector B G(A ) , with A A we have m m The relationship of differential and derivative is clarified. The Taylor expansion (P. 12) is applied to important examples, (a B) A (a B)A a (BA) if a A 0 . (38) yielding e.g. the Legendre polynomials (P. 36). The Reordering rules for products of homogeneous multivector integrability (P. 42, etc.) of multivector functions are defined are and discussed. Throughout this section a number of basic differentials and derivations are performed explicitly A B (1)r(sr) B A for r s , (39) r s s r illustrating ease and power of the calculus. ([1]-[5]). As for the notation: P. 7 refers to proposition 7 of this A B (1)rs B A . (40) r s s r section. Def. 13 refers to definition 13 of this section. (6) Elementary combinations that occur often are refers to equation number (6) in the previous section on basic geometric algebra. a (b c) (a b)c (a c)b a bc a cb, (41) Standard definitions of continuity and scalar differentiability apply to multivector-valued functions, F F((x)), (x) scalar valued function.