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EXAMPLE
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a(1)=0 triivially
a(2)=5 because the are 5 eligible numbers 11, 22, 33, 44, 55
a(3)=5 because the are 5 eligible numbers 111,.222, 333,444, 555
a(4)=65 because the are 65 eligible numbers:
1111,1122,1133,1144,1155,1212,1221,1313,1331,1414,1441,1515,1551,
2112,2121,2211,2222,2233,2244,2255,2323,2332,2424,2442,2525,2552,
3113,3131,3223,3232,3311,3322,3333,3344,3355,3434,3443,3535,3553,
4114,4141,4224,4242,4334,4343,4411,4422,4433,4444,4455,4545,4554,
5115,5151,5225,5252,5335,5353,5445,5454,5511,5522,5533,5544,5555.
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