\parindent=0pt
{\tt ;\ From\ "Quantum\ Electrodynamics"\ by\ Richard\ P.\ Feynman}

{\tt ;\ pp.\ 40-43}

$$
a=\left(\matrix{
a_t\cr
a_x\cr
a_y\cr
a_z\cr
}\right)
\quad
b=\left(\matrix{
b_t\cr
b_x\cr
b_y\cr
b_z\cr
}\right)
\quad
c=\left(\matrix{
c_t\cr
c_x\cr
c_y\cr
c_z\cr
}\right)
$$
{\tt ;\ generic\ spacetime\ vectors\ a,\ b\ and\ c}

{\tt (setq\ a\ (sum}

{\tt \ \ (product\ at\ (tensor\ t))}

{\tt \ \ (product\ ax\ (tensor\ x))}

{\tt \ \ (product\ ay\ (tensor\ y))}

{\tt \ \ (product\ az\ (tensor\ z))}

{\tt ))}

{\tt (setq\ b\ (sum}

{\tt \ \ (product\ bt\ (tensor\ t))}

{\tt \ \ (product\ bx\ (tensor\ x))}

{\tt \ \ (product\ by\ (tensor\ y))}

{\tt \ \ (product\ bz\ (tensor\ z))}

{\tt ))}

{\tt (setq\ c\ (sum}

{\tt \ \ (product\ ct\ (tensor\ t))}

{\tt \ \ (product\ cx\ (tensor\ x))}

{\tt \ \ (product\ cy\ (tensor\ y))}

{\tt \ \ (product\ cz\ (tensor\ z))}

{\tt ))}

{\tt ;\ define\ this\ function\ for\ multiplying\ spactime\ vectors}

{\tt ;\ how\ it\ works:\ (dot\ arg1\ (tensor\ t))\ picks\ off\ the\ t'th\ element,\ etc.}

{\tt ;\ the\ -1's\ are\ for\ the\ spacetime\ metric}

{\tt (define\ spacetime-dot\ (sum}

{\tt \ \ (dot\ (dot\ arg1\ (tensor\ t))\ (dot\ arg2\ (tensor\ t)))}

{\tt \ \ (dot\ -1\ (dot\ arg1\ (tensor\ x))\ (dot\ arg2\ (tensor\ x)))}

{\tt \ \ (dot\ -1\ (dot\ arg1\ (tensor\ y))\ (dot\ arg2\ (tensor\ y)))}

{\tt \ \ (dot\ -1\ (dot\ arg1\ (tensor\ z))\ (dot\ arg2\ (tensor\ z)))}

{\tt ))}

$$a^2\mathrel{\mathop=^?}a_t^2-a_x^2-a_y^2-a_z^2$$
{\tt (setq\ temp1\ (spacetime-dot\ a\ a))}

{\tt (setq\ temp2\ (sum}

{\tt \ \ (power\ at\ 2)}

{\tt \ \ (product\ -1\ (power\ ax\ 2))}

{\tt \ \ (product\ -1\ (power\ ay\ 2))}

{\tt \ \ (product\ -1\ (power\ az\ 2))}

{\tt ))}

{\tt (equal\ temp1\ temp2)\ ;\ print\ "t"\ if\ it's\ true}

$$I=\left(\matrix{
1&0&0&0\cr
0&1&0&0\cr
0&0&1&0\cr
0&0&0&1\cr
}\right)$$
{\tt (setq\ I\ (sum}

{\tt \ \ (tensor\ t\ t)}

{\tt \ \ (tensor\ x\ x)}

{\tt \ \ (tensor\ y\ y)}

{\tt \ \ (tensor\ z\ z)}

{\tt ))}

$$\gamma_t=\left(\matrix{
1&0&0&0\cr
0&1&0&0\cr
0&0&-1&0\cr
0&0&0&-1\cr
}\right)$$
{\tt (setq\ gammat\ (sum}

{\tt \ \ (tensor\ t\ t)}

{\tt \ \ (tensor\ x\ x)}

{\tt \ \ (product\ -1\ (tensor\ y\ y))}

{\tt \ \ (product\ -1\ (tensor\ z\ z))}

{\tt ))}

$$\gamma_x=\left(\matrix{
0&0&0&1\cr
0&0&1&0\cr
0&-1&0&0\cr
-1&0&0&0\cr
}\right)$$
{\tt (setq\ gammax\ (sum}

{\tt \ \ (tensor\ t\ z)}

{\tt \ \ (tensor\ x\ y)}

{\tt \ \ (product\ -1\ (tensor\ y\ x))}

{\tt \ \ (product\ -1\ (tensor\ z\ t))}

{\tt ))}

$$\gamma_y=\left(\matrix{
0&0&0&-i\cr
0&0&i&0\cr
0&i&0&0\cr
-i&0&0&0\cr
}\right)$$
{\tt (setq\ gammay\ (sum}

{\tt \ \ (product\ -1\ i\ (tensor\ t\ z))}

{\tt \ \ (product\ i\ (tensor\ x\ y))}

{\tt \ \ (product\ i\ (tensor\ y\ x))}

{\tt \ \ (product\ -1\ i\ (tensor\ z\ t))}

{\tt ))}

$$\gamma_z=\left(\matrix{
0&0&1&0\cr
0&0&0&-1\cr
-1&0&0&0\cr
0&1&0&0\cr
}\right)$$
{\tt (setq\ gammaz\ (sum}

{\tt \ \ (tensor\ t\ y)}

{\tt \ \ (product\ -1\ (tensor\ x\ z))}

{\tt \ \ (product\ -1\ (tensor\ y\ t))}

{\tt \ \ (tensor\ z\ x)}

{\tt ))}

$$\gamma_t^2\mathrel{\mathop=^?}1$$
{\tt (equal\ (dot\ gammat\ gammat)\ I)\ ;\ print\ "t"\ if\ it's\ true}

$$\gamma_x^2=\gamma_y^2=\gamma_z^2\mathrel{\mathop=^?}-1$$
{\tt (equal\ (dot\ gammax\ gammax)\ (dot\ -1\ I))\ ;\ print\ "t"\ if\ it's\ true}

{\tt (equal\ (dot\ gammay\ gammay)\ (dot\ -1\ I))\ ;\ print\ "t"\ if\ it's\ true}

{\tt (equal\ (dot\ gammaz\ gammaz)\ (dot\ -1\ I))\ ;\ print\ "t"\ if\ it's\ true}

$$\gamma_5=\gamma_x\gamma_y\gamma_z\gamma_t$$
{\tt (setq\ gamma5\ (dot\ gammax\ gammay\ gammaz\ gammat))}

$$\gamma_5^2\mathrel{\mathop=^?}-1$$
{\tt (equal\ (dot\ gamma5\ gamma5)\ (dot\ -1\ I))\ ;\ print\ "t"\ if\ it's\ true}

$$\gamma=\left(\matrix{
\gamma_t\cr
\gamma_x\cr
\gamma_y\cr
\gamma_z\cr
}\right)$$
{\tt ;\ gamma\ is\ a\ "vector"\ of\ dirac\ matrices}

{\tt (setq\ gamma\ (sum}

{\tt \ \ (product\ gammat\ (tensor\ t))}

{\tt \ \ (product\ gammax\ (tensor\ x))}

{\tt \ \ (product\ gammay\ (tensor\ y))}

{\tt \ \ (product\ gammaz\ (tensor\ z))}

{\tt ))}

$$a\!\!\!/=a\gamma\qquad b\!\!\!/=b\gamma\qquad c\!\!\!/=c\gamma$$
{\tt (setq\ agamma\ (spacetime-dot\ a\ gamma))}

{\tt (setq\ bgamma\ (spacetime-dot\ b\ gamma))}

{\tt (setq\ cgamma\ (spacetime-dot\ c\ gamma))}

$$a\!\!\!/\mathrel{\mathop=^?}a_t\gamma_t-a_x\gamma_x-a_y\gamma_y-a_x\gamma_x$$
{\tt (setq\ temp1\ agamma)}

{\tt (setq\ temp2\ (sum}

{\tt \ \ (product\ at\ gammat)}

{\tt \ \ (product\ -1\ ax\ gammax)}

{\tt \ \ (product\ -1\ ay\ gammay)}

{\tt \ \ (product\ -1\ az\ gammaz)}

{\tt ))}

{\tt (equal\ temp1\ temp2)\ ;\ print\ "t"\ if\ it's\ true}

$$a\!\!\!/b\!\!\!/\mathrel{\mathop=^?}-b\!\!\!/a\!\!\!/ + 2ab$$
{\tt ;\ note:\ gammas\ are\ square\ matrices,\ use\ "dot"\ to\ multiply}

{\tt ;\ use\ "spacetime-dot"\ to\ multiply\ spacetime\ vectors}

{\tt (setq\ temp1\ (dot\ agamma\ bgamma))}

{\tt (setq\ temp2\ (sum}

{\tt \ \ (dot\ -1\ bgamma\ agamma)}

{\tt \ \ (dot\ 2\ (spacetime-dot\ a\ b)\ I)}

{\tt ))}

{\tt (equal\ temp1\ temp2)\ ;\ print\ "t"\ if\ it's\ true}

$$a\!\!\!/\gamma_5\mathrel{\mathop=^?}-\gamma_5a\!\!\!/$$
{\tt (setq\ temp1\ (dot\ agamma\ gamma5))}

{\tt (setq\ temp2\ (dot\ -1\ gamma5\ agamma))}

{\tt (equal\ temp1\ temp2)\ ;\ print\ "t"\ if\ it's\ true}

$$\gamma_x a\!\!\!/\gamma_x\mathrel{\mathop=^?}a\!\!\!/+2a_x\gamma_x$$
{\tt (setq\ temp1\ (dot\ gammax\ agamma\ gammax))}

{\tt (setq\ temp2\ (sum\ agamma\ (dot\ 2\ ax\ gammax)))}

{\tt (equal\ temp1\ temp2)\ ;\ print\ "t"\ if\ it's\ true}

$$\gamma\gamma\mathrel{\mathop=^?}4$$
{\tt (setq\ temp1\ (spacetime-dot\ gamma\ gamma))}

{\tt (setq\ temp2\ (dot\ 4\ I))}

{\tt (equal\ temp1\ temp2)\ ;\ print\ "t"\ if\ it's\ true}

$$\gamma a\!\!\!/\gamma\mathrel{\mathop=^?}-2a\!\!\!/$$
{\tt (setq\ temp1\ (spacetime-dot\ gamma\ (dot\ agamma\ gamma)))}

{\tt (setq\ temp2\ (dot\ -2\ agamma))}

{\tt (equal\ temp1\ temp2)\ ;\ print\ "t"\ if\ it's\ true}

$$\gamma a\!\!\!/b\!\!\!/\gamma\mathrel{\mathop=^?}4ab$$
{\tt (setq\ temp1\ (spacetime-dot\ gamma\ (dot\ agamma\ bgamma\ gamma)))}

{\tt (setq\ temp2\ (dot\ 4\ (spacetime-dot\ a\ b)\ I))}

{\tt (equal\ temp1\ temp2)\ ;\ print\ "t"\ if\ it's\ true}

$$\gamma a\!\!\!/b\!\!\!/c\!\!\!/\gamma\mathrel{\mathop=^?}
-2c\!\!\!/b\!\!\!/a\!\!\!/$$
{\tt (setq\ temp1\ (spacetime-dot\ gamma\ (dot\ agamma\ bgamma\ cgamma\ gamma)))}

{\tt (setq\ temp2\ (dot\ -2\ cgamma\ bgamma\ agamma))}

{\tt (equal\ temp1\ temp2)\ ;\ print\ "t"\ if\ it's\ true}

{\tt ;\ define\ series\ approximations\ for\ some\ transcendental\ functions}

{\tt ;\ for\ 32-bit\ integers,\ overflow\ occurs\ for\ powers\ above\ 5}

{\tt (define\ order\ 5)}

{\tt (define\ yexp\ (prog\ temp\ count}

{\tt \ \ (setq\ temp\ 0)}

{\tt \ \ (setq\ count\ order)}

{\tt loop}

{\tt \ \ (setq\ temp\ (product\ (power\ count\ -1)\ arg\ (sum\ 1\ temp)))}

{\tt \ \ (setq\ count\ (sum\ count\ -1))}

{\tt \ \ (cond\ ((greaterp\ count\ 0)\ (goto\ loop)))}

{\tt \ \ (return\ (sum\ 1\ temp))}

{\tt ))}

{\tt (define\ ysin\ (sum}

{\tt \ \ (product\ -1/2\ i\ (yexp\ (product\ i\ arg)))}

{\tt \ \ (product\ 1/2\ i\ (yexp\ (product\ -1\ i\ arg)))}

{\tt ))}

{\tt (define\ ycos\ (sum}

{\tt \ \ (product\ 1/2\ (yexp\ (product\ i\ arg)))}

{\tt \ \ (product\ 1/2\ (yexp\ (product\ -1\ i\ arg)))}

{\tt ))}

{\tt (define\ ysinh\ (sum}

{\tt \ \ (product\ 1/2\ (yexp\ arg))}

{\tt \ \ (product\ -1/2\ (yexp\ (product\ -1\ arg)))}

{\tt ))}

{\tt (define\ ycosh\ (sum}

{\tt \ \ (product\ 1/2\ (yexp\ arg))}

{\tt \ \ (product\ 1/2\ (yexp\ (product\ -1\ arg)))}

{\tt ))}

{\tt ;\ same\ as\ above\ but\ for\ matrices}

{\tt (define\ YEXP\ (prog\ temp\ count}

{\tt \ \ (setq\ temp\ 0)}

{\tt \ \ (setq\ count\ order)}

{\tt loop}

{\tt \ \ (setq\ temp\ (dot\ (power\ count\ -1)\ arg\ (sum\ I\ temp)))}

{\tt \ \ (setq\ count\ (sum\ count\ -1))}

{\tt \ \ (cond\ ((greaterp\ count\ 0)\ (goto\ loop)))}

{\tt \ \ (return\ (sum\ I\ temp))}

{\tt ))}

{\tt (define\ YSIN\ (sum}

{\tt \ \ (product\ -1/2\ i\ (YEXP\ (product\ i\ arg)))}

{\tt \ \ (product\ 1/2\ i\ (YEXP\ (product\ -1\ i\ arg)))}

{\tt ))}

{\tt (define\ YCOS\ (sum}

{\tt \ \ (product\ 1/2\ (YEXP\ (product\ i\ arg)))}

{\tt \ \ (product\ 1/2\ (YEXP\ (product\ -1\ i\ arg)))}

{\tt ))}

{\tt (define\ YSINH\ (sum}

{\tt \ \ (product\ 1/2\ (YEXP\ arg))}

{\tt \ \ (product\ -1/2\ (YEXP\ (product\ -1\ arg)))}

{\tt ))}

{\tt (define\ YCOSH\ (sum}

{\tt \ \ (product\ 1/2\ (YEXP\ arg))}

{\tt \ \ (product\ 1/2\ (YEXP\ (product\ -1\ arg)))}

{\tt ))}

{\tt ;\ for\ truncating\ products\ of\ power\ series}

{\tt (define\ POWER\ (cond}

{\tt \ \ ((greaterp\ arg2\ order)\ 0)}

{\tt \ \ (t\ (list\ 'power\ arg1\ arg2))}

{\tt ))}

{\tt (define\ truncate\ (eval\ (subst\ 'POWER\ 'power\ arg)))}

$$\exp[(u/2)\gamma_t\gamma_x]\mathrel{\mathop=^?}
\cosh(u/2)+\gamma_t\gamma_x\sinh(u/2)$$
{\tt (setq\ temp1\ (YEXP\ (dot\ 1/2\ u\ gammat\ gammax)))}

{\tt (setq\ temp2\ (sum}

{\tt \ \ (product\ I\ (ycosh\ (product\ 1/2\ u)))\ ;\ could\ use\ "dot"\ but\ not\ necessary}

{\tt \ \ (dot\ gammat\ gammax\ (ysinh\ (product\ 1/2\ u)))}

{\tt ))}

{\tt (equal\ temp1\ temp2)\ ;\ print\ t\ if\ it's\ true}

$$\exp[(\theta/2)\gamma_x\gamma_y]\mathrel{\mathop=^?}
\cos(\theta/2)+\gamma_x\gamma_y\sin(\theta/2)$$
{\tt (setq\ temp1\ (YEXP\ (dot\ 1/2\ theta\ gammax\ gammay)))}

{\tt (setq\ temp2\ (sum}

{\tt \ \ (product\ I\ (ycos\ (product\ 1/2\ theta)))\ ;\ could\ use\ "dot"\ but\ not\ necessary}

{\tt \ \ (dot\ gammax\ gammay\ (ysin\ (product\ 1/2\ theta)))}

{\tt ))}

{\tt (equal\ temp1\ temp2)\ ;\ print\ t\ if\ it's\ true}

$$\exp[-(u/2)\gamma_t\gamma_z]\gamma_t
\exp[(u/2)\gamma_t\gamma_z]
\mathrel{\mathop=^?}\gamma_t\cosh u+\gamma_z\sinh u$$
{\tt (setq\ temp1\ (truncate\ (dot}

{\tt \ \ (YEXP\ (dot\ -1/2\ u\ gammat\ gammaz))}

{\tt \ \ gammat}

{\tt \ \ (YEXP\ (dot\ 1/2\ u\ gammat\ gammaz))}

{\tt )))}

{\tt (setq\ temp2\ (sum}

{\tt \ \ (product\ gammat\ (ycosh\ u))\ ;\ could\ use\ "dot"\ but\ not\ necessary}

{\tt \ \ (product\ gammaz\ (ysinh\ u))\ ;\ could\ use\ "dot"\ but\ not\ necessary}

{\tt ))}

{\tt (equal\ temp1\ temp2)\ ;\ print\ t\ if\ it's\ true}

$$\exp[-(u/2)\gamma_t\gamma_z]\gamma_z
\exp[(u/2)\gamma_t\gamma_z]
\mathrel{\mathop=^?}\gamma_z\cosh u+\gamma_t\sinh u$$
{\tt (setq\ temp1\ (truncate\ (dot}

{\tt \ \ (YEXP\ (dot\ -1/2\ u\ gammat\ gammaz))}

{\tt \ \ gammaz}

{\tt \ \ (YEXP\ (dot\ 1/2\ u\ gammat\ gammaz))}

{\tt )))}

{\tt (setq\ temp2\ (sum}

{\tt \ \ (product\ gammaz\ (ycosh\ u))\ ;\ could\ use\ "dot"\ but\ not\ necessary}

{\tt \ \ (product\ gammat\ (ysinh\ u))\ ;\ could\ use\ "dot"\ but\ not\ necessary}

{\tt ))}

{\tt (equal\ temp1\ temp2)\ ;\ print\ t\ if\ it's\ true}

$$\exp[-(u/2)\gamma_t\gamma_z]\gamma_y
\exp[(u/2)\gamma_t\gamma_z]
\mathrel{\mathop=^?}\gamma_y$$
{\tt (setq\ temp1\ (truncate\ (dot}

{\tt \ \ (YEXP\ (dot\ -1/2\ u\ gammat\ gammaz))}

{\tt \ \ gammay}

{\tt \ \ (YEXP\ (dot\ 1/2\ u\ gammat\ gammaz))}

{\tt )))}

{\tt (equal\ temp1\ gammay)\ ;\ print\ t\ if\ it's\ true}

$$\exp[-(u/2)\gamma_t\gamma_z]\gamma_x
\exp[(u/2)\gamma_t\gamma_z]
\mathrel{\mathop=^?}\gamma_x$$
{\tt (setq\ temp1\ (truncate\ (dot}

{\tt \ \ (YEXP\ (dot\ -1/2\ u\ gammat\ gammaz))}

{\tt \ \ gammax}

{\tt \ \ (YEXP\ (dot\ 1/2\ u\ gammat\ gammaz))}

{\tt )))}

{\tt (equal\ temp1\ gammax)\ ;\ print\ t\ if\ it's\ true}

\end