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%I #14 Sep 08 2022 08:46:19
%S 7,9,77,79,97,99,777,779,797,799,977,979,997,999,7777,7779,7797,7799,
%T 7977,7979,7997,7999,9777,9779,9797,9799,9977,9979,9997,9999,77777,
%U 77779,77797,77799,77977,77979,77997,77999,79777,79779,79797,79799,79977,79979
%N Numbers with digits 7 and 9 only.
%t Flatten@ Table[FromDigits /@ Tuples[{7, 9}, n], {n, 5}] (* _Giovanni Resta_, Apr 10 2017 *)
%o (Magma) [n: n in [1..100000] | Set(IntegerToSequence(n, 10)) subset {7, 9}]
%o (PARI) a(n,{p=[7,9]})={my(v=binary(n+1));fromdigits(vector(#v-1,i,p[2]*v[i+1]+p[1]*!v[i+1]))} \\ _R. J. Cano_, Apr 09 2017
%o (Python)
%o def a(n): return int(bin(n+1)[3:].replace('0', '7').replace('1', '9'))
%o print([a(n) for n in range(1, 45)]) # _Michael S. Branicky_, Jul 09 2021
%Y Prime terms are in A020471.
%Y Numbers with digits 7 and k only for k = 0 - 6 and 8 - 9: A204094 (k = 0), A276039 (k = 1), A284921 (k = 2), A143967 (k = 3), A284971 (k = 4), A284380 (k = 5), A256292 (k = 6), A256340 (k = 8), this sequence (k = 9).
%K nonn,base
%O 1,1
%A _Jaroslav Krizek_, Apr 08 2017
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