int i;

void setup() {
  pinMode(13, OUTPUT);
  // put your setup code here, to run once:
}

void loop() {
  for (int i=0; i<= 255;
  i++) {
    analogWrite(13,i);
    delay(10);
  }
  delay(100);
  for (int i = 255; i>= 0; i--) {
    analogWrite(13,i);
    delay(10);
  }
  delay(100);
  }
  // put your main code here, to run repeatedly: