Explanation:
in such cases you always look at the differences between the given values, and if you see a pattern there right away.
the difference between 1 and 2 is 1.
the difference between 2 and 6 is 4. that is 3 more than the previous difference.
the difference between 6 and 13 is 7. that is 3 more than the previous difference.
the difference between 36 and 52 is 16. that is 9 more than the last known difference. and it covers 3 items of the sequence : 9/3 = 3.
so, it seems, each new term is created by increasing the difference to the previous term by 3 compared to the difference of the previous term to the one before it.
that means the number after 13 is
13 + previous difference + 3 = 13 + 7 + 3 = 23. the difference to the previous number (13) is now 10.
then the number after 23 must be
23 + 10 + 3 = 36
that fits.
and then again
36 + 13 + 3 = 52.
so, the missing number is 23.