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A206447 Composite numbers n such that sigma(n) = sigma(d) has solution for some other composite number d. 2
14, 15, 16, 20, 24, 25, 26, 28, 30, 33, 35, 38, 39, 40, 42, 44, 46, 48, 51, 54, 55, 56, 58, 60, 62, 65, 66, 68, 69, 70, 75, 77, 78, 80, 82, 84, 87, 88, 90, 92, 94, 95, 96, 99, 102, 104, 105, 108, 110, 112, 114, 115, 116, 118, 119, 120, 122, 123, 124, 125 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Robert Israel, Table of n, a(n) for n = 1..10000

EXAMPLE

Composite numbers 14 and 15 are in sequence because sigma(14) = sigma(15) = 24.

MAPLE

N:= 500:

Res:= {}: Q:= {}:

for n from 4 to N do

  if isprime(n) then next fi;

  s:= numtheory:-sigma(n);

  if not assigned(V[s]) then

     V[s]:= n;

     if s > N then Q:= Q union {n} fi;

  else

     Res:= Res union {n, V[s]};

     if s > N then Q:= Q minus {V[s]} fi;

  fi

od:

convert(select(`<`, Res, min(Q)), list); # Robert Israel, Dec 17 2017

MATHEMATICA

t2 = Table[If[PrimeQ[n], 0, DivisorSigma[1, n]], {n, 1000}]; Select[Range[132], ! PrimeQ[#] && Length[Position[t2, t2[[#]]]] > 1 &] (* T. D. Noe, Feb 27 2012 *)

CROSSREFS

Cf. A206036, A158913, A066073.

Sequence in context: A048125 A163482 A257782 * A293792 A308974 A130687

Adjacent sequences:  A206444 A206445 A206446 * A206448 A206449 A206450

KEYWORD

nonn

AUTHOR

Jaroslav Krizek, Feb 07 2012

STATUS

approved

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Last modified July 31 02:33 EDT 2021. Contains 346367 sequences. (Running on oeis4.)