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A180162
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a(n) is the smallest number N such that sigma(N) is an n-th power but not a higher power, with a(n) = 0 if no such number exists.
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5
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1, 2, 3, 7, 510, 21, 17490, 93, 217, 381, 651, 118879530, 2667, 8191, 11811, 24573, 57337, 82677, 172011, 393213, 761763, 1572861, 2752491, 5332341, 11010027, 21845397, 48758691, 85327221, 199753347, 341310837, 677207307, 1398273429
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OFFSET
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0,2
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LINKS
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FORMULA
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EXAMPLE
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a(4)=510 since 510=2*3*5*17, sigma(510)=2^4*3^4.
a(11)=2*3*5*7*11*53*971=118879530 since sigma(118879530)=6^11.
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MAPLE
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with(numtheory);
egcd:=proc(n::posint) local L; if n>1 then L:=ifactors(n)[2]; L:=map(z-> z[2], L); igcd(op(L)) else 0 fi end:
P:={}: SP:={}:
for w to 1 do
for n from 1 to 12^6 do
sn:=sigma(n);
esn:=egcd(sn);
if not esn in P then
P:=P union {esn};
SP:=SP union {[esn, n]};
printf("n=%d, esn=%d, sn=...\n", n, esn);
print(ifactor(sn));
fi;
od; #n
od; #w
P; SP;
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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