In particular, five is the first congruent number, since it is the length of the hypotenuse of the smallest integer-sided right triangle.Five is the second Fermat prime of the form 
  
    
      
        
          2
          
            
              2
              
                n
              
            
          
        
      
    
    {\displaystyle 2^{2^{n}}}
  + 
  
    
      
        1
      
    
    {\displaystyle 1}
  , and more generally the second Sierpiński number of the first kind, 
  
    
      
        
          n
          
            n
          
        
      
    
    {\displaystyle n^{n}}
  + 
  
    
      
        1
      
    
    {\displaystyle 1}
  .
Treatment
The standard treatment for DIPG is 6 weeks of radiation therapy, which often dramatically improves symptoms. However, symptoms usually recur after 6 to 9 months and progress rapidly. Neurosurgery
Surgery to attempt tumour removal is usually not possible or advisable for DIPG. By nature, these tumors invade diffusely throughout the brain stem, growing between normal nerve cells. Aggressive surgery would cause severe damage to neural structures vital for arm and leg movement, eye movement, swallowing, breathing, and even consciousness. A neurosurgically performed brain-stem biopsy for immunotyping of diffuse intrinsic pontine glioma has served a limited recent role in experimental clinical studies and treatment trials. This, however, is not the current standard of care, as it presents considerable risk given the biopsy location, and thus is appropriately performed only in the context of participation in an ongoing clinical treatment trial. Pontine biopsy is in no way a therapeutic or curative surgery, and the risks (potentially catastrophic and fatal) are only outweighed when the diagnosis is uncertain (extremely unusual) or the patient is enrolled in an approved clinical trial. Radiotherapy
Conventional radiotherapy, limited to the involved area of tumour, is the mainstay of treatment for DIPG. A total radiation dosage ranging from 5400 to 6000 cGy, administered in daily fractions of 150 to 200 cGy over 6 weeks, is standard. Hyperfractionated (twice-daily) radiotherapy was used previously to deliver higher radiation dosages, but did not lead to improved survival. Radiosurgery (e.g., gamma knife or cyberknife) has a role in the treatment of DIPG and may be considered in selected cases.