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This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

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A159586 a(n) is the smallest number not yet used such that n*a(n) is a square, but n != a(n).

Original entry on oeis.org

4, 8, 12, 1, 20, 24, 28, 2, 16, 40, 44, 3, 52, 56, 60, 9, 68, 32, 76, 5, 84, 88, 92, 6, 36, 104, 48, 7, 116, 120, 124, 18, 132, 136, 140, 25, 148, 152, 156, 10, 164, 168, 172, 11, 80, 184, 188, 27, 64, 72, 204, 13, 212, 96, 220, 14, 228, 232, 236, 15, 244, 248, 112, 49
Offset: 1

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Author

Franklin T. Adams-Watters, Apr 16 2009

Keywords

Comments

This is a self-inverse permutation of the positive integers.
Equivalently, the geometric mean of n and a(n) is an integer.
If n is squarefree, a(n) = 4n.

Crossrefs

Cf. A159253, A056135.

Programs

  • PARI
    invect(v,n,x)=for(i=1,n,if(v[i]==x,return(1)));0
nextnew(v,n)=local(k);k=1;while(!R(n,k)||invect(v,n-1,k),k++);k
newvect(n)=local(v);v=vector(n);for(k=1,n,v[k]=nextnew(v,k));v
R(n,m)=issquare(n*m)&&n!=m
newvect(80)
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Started in 1964 by Neil J. A. Sloane | Maintained by The OEIS Foundation Inc.

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