An integer $n$ that in a given base $b$ lacks a digitaddition generator.

Consider, for example, the integer 41 in base 10. It can be expressed as 34 + 3 + 4. For 42, however, there is no such digitaddition, hence it is a self number.

If $2|b$ all odd $n < b$ will be self numbers.

Though self numbers form a small proportion of most ranges of $2b$ consecutive integers, there are infinitely many of them: The recurrence relation $S_i = (b - 2)b^{i - 1} + S_{i - 1} + (b - 2)$ (with $S_1 = b - 1$ if $2|b$ and $S_1 = b - 2$ otherwise) will give an incomplete though infinite list of self numbers.

Reference

Kaprekar, D. R. The Mathematics of New Self-Numbers. Devaiali, 1963: 19 - 20