login
A281091
Numbers beginning and ending with their digital root in decimal representation.
1
0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 99, 181, 272, 363, 454, 545, 636, 727, 818, 909, 999, 1081, 1171, 1261, 1351, 1441, 1531, 1621, 1711, 1801, 1891, 1981, 2072, 2162, 2252, 2342, 2432, 2522, 2612, 2702, 2792, 2882, 2972, 3063, 3153, 3243, 3333, 3423, 3513, 3603, 3693, 3783, 3873, 3963, 4054, 4144, 4234, 4324, 4414
OFFSET
1,3
COMMENTS
1% of the nonnegative integers are in the sequence, approximatively.
Up to 10^k there are exactly 1 + (10^k + 800)/90 terms. - Giovanni Resta, Apr 12 2017
LINKS
EXAMPLE
The digital root of 99 is 9 and "9" is the first and last digit of "99", so 99 is in the sequence.
The digital root of 100 is 1 and "1" is not the last digit of "100", so 100 is not in the sequence.
The digital root of 181 is 1 and "1" is the first and last digit of "181", so 181 is in the sequence.
Etc.
MATHEMATICA
{0} ~ Join ~ Select[Range[10^4], IntegerDigits[#][[{1, -1}]] == {1, 1} (Mod[#-1, 9] + 1) &] (* Giovanni Resta, Apr 12 2017 *)
CROSSREFS
Sequence in context: A348834 A342978 A378835 * A378837 A271569 A239138
KEYWORD
nonn,base,easy
AUTHOR
STATUS
approved