The Road to Reality Study Notes: Difference between revisions

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=== 5.4 Complex Powers ===
=== 5.4 Complex Powers ===
Returning to the ambiguity problem of multi-valuedness, it seems the best way to avoid issues is when a particular choice of $$logw$$ has been specified.  As an example, $$w^z$$ with $$z=\frac{1}{2}$$.  We can specify a rotation for $$logw$$ to achieve $$+w^\frac{1}{2}$$, then another rotation of $$logw$$ to achieve $$-w^frac{1}{2}$$.  The sign change is achieved because of the Euler formula <math>e^{πi}=-1</math>. Note the process:
Returning to the ambiguity problem of multi-valuedness, it seems the best way to avoid issues is when a particular choice of $$logw$$ has been specified.  As an example, $$w^z$$ with $$z=\frac{1}{2}$$.  We can specify a rotation for $$logw$$ to achieve $$+w^\frac{1}{2}$$, then another rotation of $$logw$$ to achieve $$-w^\frac{1}{2}$$.  The sign change is achieved because of the Euler formula <math>e^{πi}=-1</math>. Note the process:
<math>w^z=e^{zlogw}=e^{zre^{iθ}}=e^{ze^{iθ}}</math>, then specifying rotations for theta allows us to achieve either $$+w^\frac{1}{2}$$ or $$-w^\frac{1}{2}$$.
<math>w^z=e^{zlogw}=e^{zre^{iθ}}=e^{ze^{iθ}}</math>, then specifying rotations for theta allows us to achieve either $$+w^\frac{1}{2}$$ or $$-w^\frac{1}{2}$$.


Penrose notes an interesting curiosity for the quantity $$i^i$$.  We can specify <math>logi=\frac{1}{2}πi</math> because of the general relationship <math>logw=logr+iθ</math>.  If $$w=i$$, then its easy to see <math>logi=\frac{1}{2}πi</math> from noting that y is on the vertical axis in the complex plane (rotation of $$\frac{π}{2}$$).  This specification, and all rotations, amazingly achieve real number values for $$i^i$$.
Penrose notes an interesting curiosity for the quantity $$i^i$$.  We can specify <math>logi=\frac{1}{2}πi</math> because of the general relationship <math>logw=logr+iθ</math>.  If $$w=i$$, then its easy to see <math>logi=\frac{1}{2}πi</math> from noting that y is on the vertical axis in the complex plane (rotation of $$\frac{π}{2}$$).  This specification, and all rotations, amazingly achieve real number values for $$i^i$$.


We end the section with the idea of finite multiplicative groups, or [https://en.wikipedia.org/wiki/Cyclic_group cyclic groups] \BbbZ, which contain $$n$$ quantities with the property that any two can be multiplied together to get another member of the group.  As an example, Penrose gives us w^z= e^ze^i(theta+2*pi*i*n) with n=3, leading to three elements 1, XX, XX^2 with XX=e^2*pi*i/3. Note XX^3=1 and XX^-1=XX^2.  These form a cyclic group Z3 and in the complex plane, represent vertices of an equilateral triangle.  Multiplication by XX rotates the triangle through 2/3*pi anticlockwise and multiplication by XX^2 turns it through 2/3*pi clockwise.
We end the section with the idea of finite multiplicative groups, or [https://en.wikipedia.org/wiki/Cyclic_group cyclic groups] \BbbZ, which contain $$n$$ quantities with the property that any two can be multiplied together to get another member of the group.  As an example, Penrose gives us <math>w^z=e^{ze^{i(θ+2πin)}}</math> with $$n=3$$, leading to three elements $$1, ω, ω^2$$ with <math>ω=e^\frac{2πi}{3}</math>. Note $$ω^3=1$$ and $$ω^-1=ω^2$$.  These form a cyclic group Z3 and in the complex plane, represent vertices of an equilateral triangle.  Multiplication by ω rotates the triangle through $$\frac{2}{3}π$$ anticlockwise and multiplication by $$ω^2$$ turns it through $$\frac{2}{3}π$$clockwise.


== Chapter 6 Real-number calculus ==
== Chapter 6 Real-number calculus ==
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