發送短信 : SECOND-ORDER LINEAR PARTIAL DIFFERENTIAL EQUATIONS

  ____     _    _     _____    __   __   _____    
 |  _ \\  | || | ||  / ____||  \ \\/ // |  __ \\  
 | |_| || | || | || / //---`'   \ ` //  | |  \ || 
 | .  //  | \\_/ || \ \\___      | ||   | |__/ || 
 |_|\_\\   \____//   \_____||    |_||   |_____//  
 `-` --`    `---`     `----`     `-`'    -----`