# 1q = q = q1, i2 = j2 = k2 = -1, i j = k = -j i, i k = -j = – k i, j k = i = -k j. along with the…

1q = q = q1,

i2 = j2 = k2 = -1,

i j = k = -j i,

i k = -j = – k i,

j k = i = -k j.

along with the distributive laws

(q + r)s = qs + rs, q (r + s) = qr + qs,

for all q, r, s ∈ H.

(a) Compute the following quaternion products:

(i) j(2 – 3j + k)

(ii) (1 + i)(1 – 2i + j)

(iii) (1 + i – j – 3k)2

(iv) (2 + 2 i + 3 j – k )(2 – 2 i – 3 j + k)

(b) The conjugate of the quaternion q = a + b i + cj +d k is defined to be = a – bi – cj – d k. Prove that q = ||q||2 = q, where ||∙|| is the usual Euclidean norm on R4.

(c) Prove that quaternion multiplication is associative.

(d) Let q = a + bi + cj + d k ∈ H be a fixed quaternion. Show that Lq[r] = qr and Rq [r] = rq define linear transformations on the vector space E ≃ R4. Write down their 4 × 4 matrix representatives, and observe that they are not the same since quaternion multiplication is not commutative.

(e) Show that Lq and Rq are orthogonal matrices if ||q||2 = a2 + b2 + c2 + d2 = 1.

(f) We can identify a quaternion q = b i + c j + d k with zero real part, a = 0, with a vector q = (b, c, d)T ∈ R3. Show that, in this case, the quaternion product qr = q × r – q ∙ r can be identified with the difference between the cross and dot product of the two vectors. Which vector identities result from the associativity of quaternion multiplication?