A031954 Numbers with exactly two distinct base-9 digits.

9, 11, 12, 13, 14, 15, 16, 17, 18, 19, 21, 22, 23, 24, 25, 26, 27, 28, 29, 31, 32, 33, 34, 35, 36, 37, 38, 39, 41, 42, 43, 44, 45, 46, 47, 48, 49, 51, 52, 53, 54, 55, 56, 57, 58, 59, 61, 62, 63, 64, 65, 66, 67, 68, 69, 71, 72, 73, 74, 75, 76, 77, 78, 79, 81, 82, 90, 92, 93, 94, 95, 96, 97, 98, 100, 101, 109, 111

Offset: 1

Author

Clark Kimberling

Keywords

• nonn
• base
• easy

Crossrefs

Cf. A007095.

Programs

Maple

isA031954 := proc(n)
    local dgs,p,s ;
    dgs := convert(convert(n,base,9),set) ;
    if nops(dgs) = 2 then
        true ;
    else
        false ;
    end if ;
end proc:
A031954 := proc(n)
    option remember ;
    if n =1 then
        9 ;
    else
        for a from procname(n-1)+1 do
            if isA031954(a) then
                return a;
            end if;
        end do:
    end if;
end proc:
seq(A031954(n),n=1..80) ; # R. J. Mathar, Jan 24 2023

PARI

a(n)=n+8+(n+7)\9 \\ Charles R Greathouse IV, Mar 10 2021