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 A209291 Sum of the refactorable numbers less than or equal to n. 0
 1, 3, 3, 3, 3, 3, 3, 11, 20, 20, 20, 32, 32, 32, 32, 32, 32, 50, 50, 50, 50, 50, 50, 74, 74, 74, 74, 74, 74, 74, 74, 74, 74, 74, 74, 110, 110, 110, 110, 150, 150, 150, 150, 150, 150, 150, 150, 150, 150, 150, 150, 150, 150, 150, 150, 206, 206, 206, 206, 266 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS A number is refactorable if it is divisible by the number of its divisors. The first 8 terms of a(n) are odd.  The next odd term after 11 is a(225) = 2395. LINKS FORMULA a(n) = sum( (i) * ( 1 + floor(i/d(i)) - ceil(i/d(i)) ), i = 1..n ), where d(n) is the number of divisors less than or equal to n. MAPLE with(numtheory); a:=n->sum( (i) * ( 1 + floor(i/tau(i)) - ceil(i/tau(i)) ), i = 1..n ); CROSSREFS Cf. A033950, A000005, A141586, A057265, A036896, A036898, A114617. Sequence in context: A210746 A283986 A105159 * A176873 A227727 A050499 Adjacent sequences:  A209288 A209289 A209290 * A209292 A209293 A209294 KEYWORD nonn AUTHOR Wesley Ivan Hurt, Jan 16 2013 STATUS approved

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Last modified June 25 10:10 EDT 2019. Contains 324351 sequences. (Running on oeis4.)