This is the cops' thread. The robbers' thread goes here.

The last cops-and-robbers thread was already 4 months ago.

The cop's task

• The cop's task is to write a program/function that takes a positive (or non-negative) integer and outputs/returns another number (not necessarily integer).
• The cop will provide at least 2 sample inputs/outputs.
• For example, if I have decided to write the Fibonacci sequence, I would write this in my answer:

a(0) returns 0
a(3) returns 2

• The number of examples is up to the cop's discretion.
• However, the sequence must actually exist in the On-Line Encyclopedia of Integer Sequences®, so no pseudo-random number generator for you. :(
• No-ops (characters that do nothing) are not allowed
• The cop can hide as many characters as is so wished.
• For example, if my program is:

function a(n)
    if n>2 then
        return n
    else
        return a(n-1) + a(n-2)
    end
end

• Then I would hide these characters as I want:

function a(n)
    if ### then
        ########
    else
        ######################
    end
end

The robber's task

• is obviously to find the original source code.
• However, any proposed source code that produces the same set of output also counts as valid, as long as it is also found in OEIS.

Tips for the cops

• The search function in the OEIS only works for consecutive terms, so if you want to hide your sequence, then just leave a hole anywhere.
• Apparently there is no way to hide the sequence. Put this in mind when you choose the sequence.

Your score is the number of bytes in your code.

The winner will be the submission with the lowest score that hasn't been cracked in 7 days.

Only submissions that are posted in 2016 April are eligible for the win. Submissions that are posted later than this are welcome, but cannot win.

In order to claim the win you need to reveal the full code and the OEIS sequence (after 7 days).

Your post should be formatted like this (NN is the number of characters):

Lua, 98 bytes

Output:

a(0) returns 0
a(3) returns 2

Code (# marks unrevealed characters):

function a(n)
    if ### then
        ########
    else
        ######################
    end
end

If the code is cracked, insert [Cracked](link to cracker) in the header. If the submission is safe, insert "Safe" in the header and reveal the full code in your answer. Only answers that have revealed the full code will be eligible for the win.