Undulating number
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An undulating number is any number that has the form ababab... when in the base 10 number system.
It is only considered after at least three digits, because otherwise all numbers below 100 would be undulating. The first 20 such numbers are:
101, 111, 121, 131, 141, 151, 161, 171, 181, 191, 202, 212, 222, 232, 242, 252, 262, 272, 282, 292 (sequence A046075 in OEIS)
Some higher undulating numbers are: 99999, 1717171, 6363, 7777, 80808
Below 10,000 there are exactly 180 undulating numbers.
