A206447 - OEIS

A206447

Composite numbers n such that sigma(n) = sigma(d) has solution for some other composite number d.

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;

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