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A284294 Numbers using only digits 1 and 9. 3

%I

%S 1,9,11,19,91,99,111,119,191,199,911,919,991,999,1111,1119,1191,1199,

%T 1911,1919,1991,1999,9111,9119,9191,9199,9911,9919,9991,9999,11111,

%U 11119,11191,11199,11911,11919,11991,11999,19111,19119,19191,19199,19911,19919

%N Numbers using only digits 1 and 9.

%C Product of digits of n is a power of 9; subsequence of A284295.

%C Prime terms are in A020457.

%F The sum of first 2^n terms is (5*20^n + 38*10^n - 95*2^n + 1420)/171. _Giovanni Resta_, Mar 25 2017

%t Join @@ (FromDigits /@ Tuples[{1,9}, #] & /@ Range[5]) (* _Giovanni Resta_, Mar 25 2017 *)

%o (MAGMA) [n: n in [1..20000] | Set(IntegerToSequence(n, 10)) subset {1, 9}]

%Y Cf. Numbers using only digits 1 and k for k = 0 and k = 2 - 9: A007088 (k = 0), A007931 (k = 2), A032917 (k = 3), A032822 (k = 4) , A276037 (k = 5), A284293 (k = 6), A276039 (k = 7), A213084 (k = 8), this sequence (k = 9).

%K nonn,base

%O 1,2

%A _Jaroslav Krizek_, Mar 25 2017

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Last modified March 30 12:10 EDT 2020. Contains 333125 sequences. (Running on oeis4.)