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 A298977 Base-7 complementary numbers: n equals the product of the 7 complement (7-d) of its base-7 digits d. 3
 12, 84, 120, 588, 840, 4116, 5880, 28812, 41160, 201684, 288120, 1411788, 2016840, 9882516, 14117880, 69177612, 98825160, 484243284, 691776120, 3389702988, 4842432840, 23727920916, 33897029880, 166095446412, 237279209160, 1162668124884, 1660954464120 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The only primitive terms of the sequence, i.e., not equal to 7 times a smaller term, are a(1) = 12 and a(3) = 120. See A294090 for the base-10 variant, which is the main entry, and A298976 for the base-6 variant. LINKS Colin Barker, Table of n, a(n) for n = 1..1000 Index entries for linear recurrences with constant coefficients, signature (0, 7). FORMULA a(n+2) = 7 a(n) for all n >= 1. From Colin Barker, Feb 10 2018: (Start) G.f.: 12*x*(1 + 7*x + 3*x^2) / (1 - 7*x^2). a(n) = 12*7^(n/2) for n>1 and even. a(n) = 120*7^((n-3)/2) for n>1 and odd. (End) EXAMPLE Denoting xyz the base-7 expansion (of n = x*7^2 + y*7 + z), we have: 12 = 15 = (7-1)*(7-5), therefore 12 is in the sequence. 84 = 150 = (7-1)*(7-5)*(7-0), therefore 84 is in the sequence. 120 = 231 = (7-2)*(7-3)*(7-1), therefore 120 is in the sequence. Since the expansion of 7*x in base 7 is that of x with a 0 appended, if x is in the sequence, then 7*x = x*(7-0) is in the sequence. PROG (PARI) is(n, b=7)={n==prod(i=1, #n=digits(n, b), b-n[i])} (PARI) a(n)=[84, 120][n%2+(n>1)]*7^(n\2-1) (PARI) Vec(12*x*(1 + 7*x + 3*x^2) / (1 - 7*x^2) + O(x^60)) \\ Colin Barker, Feb 10 2018 CROSSREFS Cf. A294090, A298976. Sequence in context: A165127 A213347 A075476 * A213784 A085409 A303916 Adjacent sequences:  A298974 A298975 A298976 * A298978 A298979 A298980 KEYWORD nonn,base,easy AUTHOR M. F. Hasler, Feb 09 2018 EXTENSIONS More terms from Colin Barker, Feb 10 2018 STATUS approved

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Last modified June 2 12:56 EDT 2020. Contains 334772 sequences. (Running on oeis4.)