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A066469 Numbers having exactly four anti-divisors. 1
13, 18, 40, 41, 61, 84, 100, 169, 289, 421, 784, 1024, 1104, 1296, 3121, 5776, 9216, 12544, 12769, 13924, 16129, 17956, 24649, 32761, 33024, 35344, 36721, 36864, 38809, 71821, 75076, 106261, 110224, 119716, 135721, 147456, 167281, 175561, 199081, 232324, 237169 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

See A066272 for definition of anti-divisor.

LINKS

Chai Wah Wu, Table of n, a(n) for n = 1..500

Jon Perry, The Anti-Divisor

Jon Perry, The Anti-divisor [Cached copy]

Jon Perry, The Anti-divisor: Even More Anti-Divisors [Cached copy]

MAPLE

A066469:= proc(q)

local k, n, t;

for n from 1 to q do

t:=0; for k from 2 to n-1 do if abs((n mod k)-k/2)<1 then t:=t+1; fi; od;

if t=4 then print(n); fi; od; end:

A066469(10^10); # Paolo P. Lava, Feb 22 2013

MATHEMATICA

antid[n_] := Select[ Union[ Join[ Select[ Divisors[2n - 1], OddQ[ # ] && # != 1 & ], Select[ Divisors[2n + 1], OddQ[ # ] && # != 1 & ], 2n/Select[ Divisors[ 2*n], OddQ[ # ] && # != 1 &]]] }, # < n & ]]; Select[ Range[10^5], Length[ antid[ # ]] == 4 & ]

PROG

(Python)

from sympy.ntheory.factor_ import antidivisor_count

A066469_list = [n for n in range(1, 10**3) if antidivisor_count(n) == 4] # Chai Wah Wu, Aug 02 2015

CROSSREFS

Cf. A066272.

Sequence in context: A317771 A037158 A318080 * A318348 A197704 A218626

Adjacent sequences:  A066466 A066467 A066468 * A066470 A066471 A066472

KEYWORD

nonn

AUTHOR

Robert G. Wilson v, Jan 02 2002

EXTENSIONS

More terms from Chai Wah Wu, Aug 02 2015

STATUS

approved

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Last modified July 10 23:30 EDT 2020. Contains 335600 sequences. (Running on oeis4.)