int ledPin = 13; 
void setup() {
  pinMode(ledPin, OUTPUT); 
}
void loop() {
  for (int i = 1; i <= 100; i++) {
    for (int j = 1; j <= 10; j++) {
      digitalWrite(ledPin, HIGH); 
      delayMicroseconds(i * 10); 
      digitalWrite(ledPin, LOW); 
      delayMicroseconds((100 - i) * 10); 
    }
  }
  for (int i = 100; i >= 1; i--) {
    for (int j = 1; j <= 10; j++) {
      digitalWrite(ledPin, HIGH); 
      delayMicroseconds(i * 10); 
      digitalWrite(ledPin, LOW); 
      delayMicroseconds((100 - i) * 10); 
    }
  }
}