anchor.diff:  $$$
   x \mapsto e^x\mathrm{K}_1(x)
   
  +#### Struve functions
  +
  +Struve functions are solutions $y(x)$ of the non-homogeneous Bessel's differential equation
  +
  +$$$
  +x^2\frac{\mathrm{d}^2y}{\mathrm{d}x^2}+x\frac{\mathrm{d}y}{\mathrm{d}x}+(x^2-\alpha^2)y = \frac{4(\frac{x}{2})^{\alpha+1}}{\sqrt{\pi}\Gamma(\alpha+\frac{1}{2})}
  +
   
   #### Modified Struve functions
   
  -Struve functions are solutions $y(x)$ of the non-homogeneous Bessel's differential equation
  -
  -$$$
  -x^2\frac{\mathrm{d}^2y}{\mathrm{d}x^2}+x\frac{\mathrm{d}y}{\mathrm{d}x}+(x^2-\alpha^2)y = \frac{4(\frac{x}{2})^{\alpha+1}}{\sqrt{\pi}\Gamma(\alpha+\frac{1}{2})}
  +Modified equation:
  +
  +$$$
  +x^2\frac{\mathrm{d}^2y}{\mathrm{d}x^2}+x\frac{\mathrm{d}y}{\mathrm{d}x}-(x^2+\alpha^2)y = \frac{4(\frac{x}{2})^{\alpha+1}}{\sqrt{\pi}\Gamma(\alpha+\frac{1}{2})}
   
   Modified Struve functions:
   
  
positive.code: $$$
  (a,b) \mapsto \sqrt{a^2 + b^2}
  
  *)
  
negative.code: Bessel and Struve Functions
  ---------------------------
  
  #### Bessel functions
  
  Bessel functions are canonical solutions $y(x)$ of Bessel's differential equation
  
  $$$
  x^2\frac{\mathrm{d}^2y}{\mathrm{d}x^2}+x\frac{\mathrm{d}y}{\mathrm{d}x}+(x^2-\alpha^2)y = 0
  
  #### Modified Bessel functions
  
  Modified Bessel's equation:
  
  $$$
  x^2\frac{\mathrm{d}^2y}{\mathrm{d}x^2}+x\frac{\mathrm{d}y}{\mathrm{d}x}-(x^2+\alpha^2)y = 0
  
  Modified Bessel functions:
  
  