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A247022
Integers m such that there is exactly one k < m with sigma(k)/k > sigma(m)/m, sigma(m) being the sum of the divisors of m.
2
3, 8, 18, 30, 72, 168, 420, 3360, 7560, 12600, 20160, 30240, 32760, 50400, 65520, 83160, 131040, 221760, 831600, 1081080, 1663200, 1801800, 2882880, 6486480, 12252240, 24504480, 41081040, 43243200, 68468400, 82162080, 136936800, 205405200, 245044800, 410810400
OFFSET
1,1
COMMENTS
Integers such that A247015(n) = 1.
EXAMPLE
sigma(8)/8 is greater than all sigma(x)/x when x < 8 except 6; so 8 is here.
MAPLE
M1:= 3/2: M2:= 1: c1:= 1:
Res:= NULL: count:= 0:
for n from 3 while count < 20 do
v:= numtheory:-sigma(n)/n;
if v > M1 then M2:= M1; M1:= v; c1:= 1
elif v = M1 then
c1:= c1+1
elif c1 = 1 and v >= M2 then
M2:= v;
Res:= Res, n: count:= count+1
fi
od:
Res; # Robert Israel, Jul 28 2020
PROG
(PARI) lista(nn) = {my(t=1, x=3/2, y); for(m=3, nn, if((g=sigma(m)/m)>x, t=1; y=x; x=g, if(g==x, t=0, if(g>=y&&t, y=g; print1(m, ", "))))); } \\ Jinyuan Wang, Jul 28 2020
CROSSREFS
Cf. A000203 (sigma), A004394 (superabundant), A017665 and A017666 (sigma(n)/n).
Cf. A247015.
Sequence in context: A288249 A360139 A004210 * A119881 A184636 A075342
KEYWORD
nonn
AUTHOR
Michel Marcus, Sep 09 2014
EXTENSIONS
a(15)-a(21) from Robert Israel, Jun 08 2018
Corrected and name changed by Robert Israel, Jul 28 2020
More terms from Jinyuan Wang, Jul 28 2020
STATUS
approved