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A005100
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Deficient numbers: numbers k such that sigma(k) < 2k.
(Formerly M0514)
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202
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1, 2, 3, 4, 5, 7, 8, 9, 10, 11, 13, 14, 15, 16, 17, 19, 21, 22, 23, 25, 26, 27, 29, 31, 32, 33, 34, 35, 37, 38, 39, 41, 43, 44, 45, 46, 47, 49, 50, 51, 52, 53, 55, 57, 58, 59, 61, 62, 63, 64, 65, 67, 68, 69, 71, 73, 74, 75, 76, 77, 79, 81, 82, 83, 85, 86
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OFFSET
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1,2
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COMMENTS
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A number k is abundant if sigma(k) > 2k (cf. A005101), perfect if sigma(k) = 2k (cf. A000396), or deficient if sigma(k) < 2k (this sequence), where sigma(k) is the sum of the divisors of k (A000203).
According to Deléglise (1998), the abundant numbers have natural density 0.2474 < A(2) < 0.2480. Since the perfect numbers have density 0, the deficient numbers have density 0.7520 < 1 - A(2) < 0.7526. Thus the n-th deficient number is asymptotic to 1.3287*n < n/(1 - A(2)) < 1.3298*n. - Daniel Forgues, Oct 10 2015
The data begins with 3 runs of 5 consecutive terms, from 1 to 5, 7 to 11 and 13 to 17. The maximal length of a run of consecutive terms is 5 because 6 is a perfect number and its proper multiples are abundant numbers. - Bernard Schott, May 19 2019
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REFERENCES
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Richard K. Guy, Unsolved Problems in Number Theory, 3rd Edition, Springer, 2004, Section B2, pp. 74-84.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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Eric Weisstein's World of Mathematics, Abundance.
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FORMULA
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MAPLE
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with(numtheory); s := proc(n) local i, j, ans; ans := [ ]; j := 0; for i while j<n do if sigma(i)<2*i then ans := [ op(ans), i ]; j := j+1; fi; od; RETURN(ans); end; # s(k) returns terms of sequence through k
isA005100 := proc(n)
numtheory[sigma](n) < 2*n ;
end proc:
option remember;
local a;
if n = 1 then
1;
else
for a from procname(n-1)+1 do
if isA005100(a) then
return a;
end if;
end do:
end if;
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MATHEMATICA
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PROG
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(PARI) for(n=1, 100, if(sigma(n) < 2*n, print1(n", "))) \\ Altug Alkan, Oct 15 2015
(Haskell)
a005100 n = a005100_list !! (n-1)
a005100_list = filter (\x -> a001065 x < x) [1..]
(Python)
from sympy import divisors
def ok(n): return sum(divisors(n)) < 2*n
(Python)
from sympy import divisor_sigma
from itertools import count, islice
def A005100_gen(startvalue=1): return filter(lambda n:divisor_sigma(n) < 2*n, count(max(startvalue, 1))) # generator of terms >= startvalue
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CROSSREFS
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By definition, the weird numbers A006037 are not in this sequence.
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KEYWORD
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nonn,easy,core,nice
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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