On the other hand, the square of four 42, equivalently the fourth power of two 24, is sixteen; the only number that has 
  
    
      
        
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   = 
  
    
      
        
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   as a form of factorization.
Signs and symptoms
FHM signs overlap significantly with those of migraine with aura. In short, FHM is typified by migraine with aura associated with hemiparesis, and in FHM1, cerebellar degeneration, which can result in episodic or progressive ataxia. FHM can also present with the same signs as benign familial infantile convulsions and alternating hemiplegia of childhood. Other symptoms are altered consciousness (in fact, some cases seem related to head trauma), gaze-evoked nystagmus, and coma. Aura symptoms, such as numbness and blurring of vision, typically persist for 30–60 minutes, but can last for weeks to months. An attack resembles a stroke, but unlike a stroke, it resolves in time. These signs typically first manifest themselves in the first or second decade of life. Causes
See the equivalent section in the main migraine article. FHM mutations are believed to lead to migraine susceptibility by lowering the threshold for cortical-spreading-depression generation. The FHM1 and FHM3 mutations occur in ion channels expressed in neurons. These mutations may lead to both the hyper- and hypoexcitable neurons that might underlie cortical-spreading-depression. How the mutations seen in FHM2 patients might lead to FHM symptoms is even less clear, as the gene mutated in FHM2 is expressed primarily in astrocytes. One proposal states that the depolarization of astrocytes caused by haploinsufficiency of the ATP1A2 Na+/K+-ATPase causes increased release of compounds such as adenosine from astrocytes. These compounds then interact with neighboring neurons, altering their excitability and leading to cortical-spreading-depression and migraine.