const int a[]={13,12,11,10,9,8,7,6,5,4,3,2};
void setup() {
  // put your setup code here, to run once:
for(int i = 0; i < sizeof(a) / sizeof(a[0]); i++) {
    pinMode(a[i], OUTPUT);
  }
}
void loop() {
  for (int j = 0; j <= 255; j++) {
    for (int i = 0; i < sizeof(a) / sizeof(a[0]); i++) {
      analogWrite(a[i], j);
      delay(10);
    }
  }
}