Algoritm 1

Algorithm ceFace(n)
  If n = 1 then Return False EndIf
  If n = 2 then Return True EndIf
  If n MOD 2 = 0 AND n ≥ 4 then Return False EndIf
  For index ← 3, index * index ≤ n execute
    If n MOD index = 0 then Return False EndIf
  EndFor
  Return True
EndAlgorithm