This is a Fibonacci series. The sum of the previous two digits gives the number. So 13

0+1=1 1+1=2 1+2=3 2+3=5 3+5=8 5+8=13

Answer is 13

Answer is 13. 1+2 = 3 2+3 = 5 3+5 =8 5+8 = 13

1+2=3 2+3=5 3+5=8 5+8=13 next number is 13

On behalf of Sture Sjöstedt