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One way to answer this question is from first principles. Let

q = a + bi + cj + dk

and

v = e + fi + gj + hk

Then,

qv

= ae - bf - cg - dh

+ i(af + be + ch - dg)

+ j(ag - bh + ce + df)

+ k(ah + bg - cf + de)

= ae - bf - cg - dh + afi + bei + agj +cej + ahk + dek

+ i(ch - dg)

+ j(df - bh)

+ k(bg - cf)

and (keeping in mind that reals commute under multiplication)

vq

= ae - bf - cg - dh

+ i(af + be - ch + dg)

+ j(ag + bh + ce - df)

+ k(ah - bg + cf + de)

= ae - bf - cg - dh + afi + bei + agj + cej + ahk + dek

- i(ch-dg)

- j(df-bh)

- k(bg-cf)

Let m = ae - bf - cg - dh + afi + bei + agj +cej + ahk + dek and n = i(ch - dg) + j(df - bh) + k(bg - cf). Then,

qv = m + n

and

vq = m - n.

For qv to equal vq, we must have n = 0. For n to equal zero, each of its parts must equal zero also, that is,

ch = dg

df = bh

bg = cf

so that either

c/g = d/h

d/h = b/f

b/f = c/g

i.e. (bi + cj + dk) is a multiple (not necessarily integer) of (fi + gj + hk)

or

b = 0, c = 0, d = 0,

or

f = 0, g = 0, h = 0.

Therefore, the nonreal part of v must be a multiple (possibly zero) of the nonreal part of q. More concisely, quaternions q and v commute under multiplication if and only if

v = pq + r

where p and r are real.

A neater proof, requiring more background, can also be given.

Quaternions are often regarded as the combination of a scalar part (the real part of the quaternion) and the vector part (the part containing i, j, and k.) That is, the quaternion a + bi + cj + dk corresponds to the combination of the scalar a and the vector . Now, let q = a + b and v = c + d (a being the scalar part of q and b the vector part, and similarly for v). Then, the product qv is given by

qv = ac + ad + bc - b·d + b×d.

In the product qv, the order of the factors in each of these five products is reversed. That is, the product is now

vq = ca + da + cb - d·b + d×b.

The first four products commute, whereas the last (a cross product) does not. Therefore,

vq = ac + ad + bc - b·d + d×b.

so that qv = vq if and only if b×d = d×b. It is known that the cross product is anticommutative, meaning that b×d = -d×b. Therefore, this condition reduces to b×d = θ, the zero vector, meaning that one is a scalar multiple of the other (i.e. they lie along the same line.) This gives the same result as the purely algebraic derivation above.

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