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A159907
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Numbers n with half-integral abundancy index, sigma(n)/n = k+1/2 with integer k.
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25
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2, 24, 4320, 4680, 26208, 8910720, 17428320, 20427264, 91963648, 197064960, 8583644160, 10200236032, 21857648640, 57575890944, 57629644800, 206166804480, 17116004505600, 1416963251404800, 15338300494970880
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OFFSET
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1,1
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COMMENTS
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Obviously, all terms must be even (cf. formula), but e.g. a(9) and a(12) are not divisible by 3. See A007691 for numbers with integral abundancy.
Odd numbers and higher powers of 2 cannot be in the sequence; 6 is in A000396 and thus in A007691, and n=10,12,14,18,20,22 don't have integral 2*sigma(n)/n.
Conjecture: with number 1, multiply-anti-perfect numbers m: m divides antisigma(m) = A024816(m). Sequence of fractions antisigma(m) / m: {0, 0, 10, 2157, 2337, 13101, 4455356, ...}. - Jaroslav Krizek, Jul 21 2011
The above conjecture is equivalent to the conjecture that there are no odd multiply perfect numbers (A007691) greater than 1. Proof: (sigma(n)+antisigma(n))/n = (n+1)/2 for all n. If n is even then sigma(n)/n is a half-integer if and only if antisigma(n)/n is an integer. Since all members of this sequence are known to be even, the only way the conjecture can fail is if antisigma(n)/n is an integer, in which case sigma(n)/n is an integer as well. - Nathaniel Johnston, Jul 23 2011
These numbers are called hemiperfect numbers. See Numericana & Wikipedia links. - Michel Marcus, Nov 19 2017
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LINKS
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FORMULA
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EXAMPLE
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a(1) = 2 since sigma(2)/2 = (1+2)/2 = 3/2 is of the form k+1/2 with integer k=1.
a(2) = 24 is in the sequence since sigma(24)/24 = (1+2+3+4+6+8+12+24)/24 = (24+12+24)/24 = k+1/2 with integer k=2.
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PROG
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(PARI) isok(n) = denominator(sigma(n, -1)) == 2; \\ Michel Marcus, Sep 19 2015
(PARI) forfactored(n=1, 10^7, if(denominator(sigma(n, -1))==2, print1(n[1]", "))) \\ Charles R Greathouse IV, May 09 2017
(Python)
from fractions import Fraction
from sympy import divisor_sigma as sigma
def aupto(limit):
for k in range(1, limit):
if Fraction(int(sigma(k, 1)), k).denominator == 2:
print(k, end=", ")
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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