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 A036897 Square root of odd refactorable numbers. 5
 1, 3, 15, 21, 25, 33, 39, 45, 51, 57, 69, 75, 81, 87, 93, 111, 123, 129, 141, 159, 177, 183, 189, 201, 213, 219, 225, 237, 249, 267, 291, 303, 309, 315, 321, 327, 339, 343, 381, 393, 405, 411, 417, 447, 453, 471, 489, 495, 501, 519, 525, 537, 543, 567, 573 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Odd refactorable numbers are always squares. LINKS Amiram Eldar, Table of n, a(n) for n = 1..1001 S. Colton, Refactorable Numbers - A Machine Invention, J. Integer Sequences, Vol. 2, 1999, #2. FORMULA a(n) = sqrt(A036896(n)). - Amiram Eldar, Jul 02 2019 EXAMPLE 15^2 is refactorable because 225 has 9 divisors and 9 divides 225. MATHEMATICA Select[Range[1, 1000, 2], Divisible[#^2, DivisorSigma[0, #^2]] &] (* Amiram Eldar, Jul 02 2019 *) PROG (PARI) isrefac(n) = ! (n % numdiv(n)); lista(nn) = {forstep (n = 1, nn, 2, if (isrefac(n), print1(sqrtint(n), ", ")); ); } \\ Michel Marcus, Aug 31 2013 CROSSREFS Cf. A033950, A036896. Sequence in context: A316751 A001897 A074214 * A129966 A243128 A306771 Adjacent sequences:  A036894 A036895 A036896 * A036898 A036899 A036900 KEYWORD nonn AUTHOR Simon Colton (simonco(AT)cs.york.ac.uk) STATUS approved

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Last modified October 23 10:07 EDT 2019. Contains 328345 sequences. (Running on oeis4.)