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 A220221 Odd positive integers k such that k^2 has at most three nonzero binary digits. 1
 1, 3, 5, 7, 9, 17, 23, 33, 65, 129, 257, 513, 1025, 2049, 4097, 8193, 16385, 32769, 65537, 131073, 262145, 524289, 1048577, 2097153, 4194305, 8388609, 16777217, 33554433, 67108865, 134217729, 268435457, 536870913, 1073741825, 2147483649, 4294967297 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS It is shown in the Szalay reference that if y is a term of this sequence then y=7, y=23, or y=2^t+1 for some positive t. Also see the Bennett reference. LINKS Colin Barker, Table of n, a(n) for n = 1..1000 Michael A. Bennett, Perfect powers with few ternary digits, INTEGERS 12A (2012), #A3. László Szalay, The equations 2^n+-2^m+-2^l=z^2, Indag. Math. 13 (2002) 131-142. Index entries for linear recurrences with constant coefficients, signature (3,-2). FORMULA a(n) = 3*a(n-1)-2*a(n-2) for n>9. - Colin Barker, Nov 06 2014 G.f.: x*(12*x^8-2*x^7-10*x^6+4*x^5-2*x^4-2*x^3-2*x^2+1) / ((x-1)*(2*x-1)). - Colin Barker, Nov 06 2014 MATHEMATICA Select[Range[1, 1000000, 2], Total[IntegerDigits[#^2, 2]] <= 3 &] (* T. D. Noe, Dec 07 2012 *) CoefficientList[Series[(12 x^8 - 2 x^7 - 10 x^6 + 4 x^5 - 2 x^4 - 2 x^3 - 2 x^2 + 1) / ((x - 1) (2 x - 1)), {x, 0, 50}], x] (* Vincenzo Librandi, Nov 07 2014 *) PROG (PARI) is(n)=n%2 && hammingweight(n^2)<4 \\ Charles R Greathouse IV, Dec 10 2012 (PARI) Vec(x*(12*x^8-2*x^7-10*x^6+4*x^5-2*x^4-2*x^3-2*x^2+1)/((x-1)*(2*x-1)) + O(x^100)) \\ Colin Barker, Nov 06 2014 (MAGMA) I:=[1, 3, 5, 7, 9, 17, 23, 33, 65, 129]; [n le 10 select I[n] else 3*Self(n-1)-2*Self(n-2): n in [1..50]]; // Vincenzo Librandi, Nov 07 2014 CROSSREFS Cf. A212191 (exactly 3 powers). Sequence in context: A084229 A191356 A144753 * A212292 A270837 A057482 Adjacent sequences:  A220218 A220219 A220220 * A220222 A220223 A220224 KEYWORD nonn,base,easy AUTHOR John W. Layman, Dec 07 2012 EXTENSIONS Extended by T. D. Noe, Dec 07 2012 STATUS approved

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Last modified July 18 07:10 EDT 2019. Contains 325134 sequences. (Running on oeis4.)