The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A280634 Number of partitions of 2n into two refactorable parts. 3
 1, 1, 0, 0, 2, 0, 1, 1, 1, 2, 0, 1, 2, 0, 1, 1, 0, 2, 1, 0, 2, 1, 0, 3, 0, 1, 1, 0, 2, 1, 1, 2, 0, 2, 0, 2, 2, 1, 1, 3, 1, 2, 1, 1, 2, 3, 0, 5, 2, 2, 1, 2, 2, 3, 1, 4, 1, 4, 0, 5, 1, 2, 1, 3, 1, 3, 1, 3, 1, 5, 0, 7, 1, 3, 1, 3, 2, 3, 1, 5, 0, 6, 0, 7, 1, 3, 1, 5, 0, 3 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,5 LINKS Eric Weisstein's World of Mathematics, Refactorable Number FORMULA a(n) = Sum_{i=1..n} (1-sign(i mod d(i))) * (1-sign((2n-i) mod d(2n-i))) where d(n) is the number of divisors of n. EXAMPLE a(5) = 2; There are two partitions of 2*5 = 10 into two refactorable parts: (1,9) and (2,8). MAPLE with(numtheory): A280634:=n->add((1-signum((i mod tau(i))))*(1-signum((2*n-i) mod tau(2*n-i))), i=1..n): seq(A280634(n), n=1..150); MATHEMATICA Table[Sum[(1 - Sign[Mod[i, DivisorSigma[0, i]]]) (1 - Sign[Mod[#, DivisorSigma[0, #]]] &[2 n - i]), {i, n}], {n, 90}] (* Michael De Vlieger, Jan 07 2017 *) CROSSREFS Cf. A000005, A033950, A172398, A280226. Sequence in context: A053692 A286934 A282714 * A281491 A099494 A030341 Adjacent sequences:  A280631 A280632 A280633 * A280635 A280636 A280637 KEYWORD nonn,easy AUTHOR Wesley Ivan Hurt, Jan 06 2017 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified February 19 13:03 EST 2020. Contains 332044 sequences. (Running on oeis4.)