A291269 Numbers that contain exactly 2 pairs of identical digits.

1001, 1010, 1100, 1122, 1133, 1144, 1155, 1166, 1177, 1188, 1199, 1212, 1221, 1313, 1331, 1414, 1441, 1515, 1551, 1616, 1661, 1717, 1771, 1818, 1881, 1919, 1991, 2002, 2020, 2112, 2121, 2200, 2211, 2233, 2244, 2255, 2266, 2277, 2288, 2299, 2323, 2332, 2424, 2442

Offset

1,1

Comments

Since other digits may be present, the last term is 998876543210; otherwise the last term would be 9988.

No multiples above 2 of identical digits are allowed, e.g., 1122333 is not a member of the sequence. - Harvey P. Dale, Nov 07 2021

Links

Harvey P. Dale, Table of n, a(n) for n = 1..4000

Mathematica

Select[Range[10^3, 10^4 - 1], Union@ Map[Length, #] == {2} &@ Split@ Sort@ IntegerDigits@ # &] (* Michael De Vlieger, Aug 24 2017 *)
(* program above only works for numbers of 4 digits, or *)
ok[n_] := Block[{d = DigitCount[n]}, Count[d, 2] == 2 && {} == Select[d, # > 2 &]]; Select[Range[11000], ok] (* Giovanni Resta, Aug 25 2017 *)
tpidQ[n_]:=Module[{dc=DigitCount[n]}, Max[dc]==2&&Count[dc, 2]==2]; Select[ Range[2500], tpidQ] (* Harvey P. Dale, Nov 07 2021 *)

Crossrefs

Sequence in context: A282985 A262859 A044881 * A317291 A362921 A241946

Adjacent sequences: A291266 A291267 A291268 * A291270 A291271 A291272

Keyword

nonn,base,fini

Author

Enrique Navarrete, Aug 21 2017

Status

approved