Iteration function (basic math) in arduino

legend:
Yi = Y new
Yi-1 = Y old

i want to make this in my arduino

my function Yi = Yi-1 +6
start y0 = 0

Y1 = Y0 +6 = 6
Y2 = Y1 +6 = 12
Y3 = Y2 +6 = 18
Y4 = Y3 +6 = 24

pelase help me to make this code, i want to make infinity iteration

i need (6, 12, 18, 24) show in my serial monitor

i’ve tried make this code but im fail

volatile double y = 0;

void setup() {
  // put your setup code here, to run once:
Serial.begin (9600);
}

void loop() {
   for (int i=1; i > 0; i++)
      { 
         y[i] += y[i-1] + 6;
        }

}

Please post the program you have now (using code tags) and explain where you are having the difficulty.