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A241646 Numbers m such that the GCD of the x's that satisfy sigma(x)=m is 1. 5
1, 12, 18, 24, 31, 32, 42, 48, 54, 56, 60, 72, 80, 84, 90, 96, 98, 104, 108, 114, 120, 128, 132, 140, 144, 152, 156, 168, 180, 182, 192, 216, 224, 228, 234, 240, 248, 252, 264, 270, 272, 280, 288, 294, 308, 312, 324, 336, 342, 360, 372, 384, 390, 408, 420 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

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

EXAMPLE

We have sigma(6)=sigma(11)=12, and gcd(6, 11) = 1, hence 12 is in the sequence.

For x in [20, 26, 41], sigma(x)=42, and gcd(20, 26, 41)=1, hence 42 is here.

MAPLE

N:= 10^4: # for terms <= N

V:= Vector(N):

for x from 1 to N do

  s:= numtheory:-sigma(x);

  if s <= N then

    if V[s] = 0 then V[s]:= x

    else V[s]:= igcd(V[s], x)

    fi

  fi

od: select(t -> V[t]=1, [$1..N]);  # Robert Israel, Aug 18 2019

CROSSREFS

Cf. A000203, A240667, A241625, A241646, A241647, A241648, A241649, A241650.

Sequence in context: A091013 A159886 A258914 * A181941 A206449 A241852

Adjacent sequences:  A241643 A241644 A241645 * A241647 A241648 A241649

KEYWORD

nonn

AUTHOR

Michel Marcus, Apr 26 2014

STATUS

approved

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Last modified May 11 13:41 EDT 2021. Contains 343791 sequences. (Running on oeis4.)