I suspect here:

    ... 48, 4a, 4d, 4g‏, 4n, 51, 5n, 5p, ...

you mean:

    ... 48, 4a, 4d, 4g, 4n, 51, 5n, 5p, ...

If you would care to be more accurate in your question then people might take more of an interest.

Also, would you care to provide some context? Is this in base 36? Are they semi-base64? What have you tried so far? What do you mean by "a formula"? What is your purpose?

In short, perhaps you could be a little clearer.