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A211185 Numbers whose number of proper divisors equals the number of their anti-divisors. 1
1, 3, 9, 10, 14, 15, 21, 26, 28, 34, 51, 69, 75, 76, 88, 92, 99, 102, 104, 106, 110, 124, 134, 135, 136, 138, 141, 146, 164, 170, 231, 232, 236, 256, 258, 261, 268, 285, 290, 309, 321, 328, 386, 394, 405, 411, 424, 429, 441, 484, 490, 525, 531, 574, 580, 590, 602, 608, 614, 615, 620, 628, 639, 645, 651, 656, 658 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

See A066272 for definition of anti-divisor.

Numbers of divisors of n such that number of proper divisors of n equals the number of anti-divisors of n: 1, 2, 2, 3, 4, 4, 4, 4, 6, 4, 4, 4, 6, 6, 4, 4, 4, 12, 4, 6, 10, 4, 8, 8, 4, 12, 4, 6, 4, 12, 4, 4, 4,...

Primes p such that number of proper divisors of p - 1 equals the number of anti-divisors of p - 1 and number of proper divisors of p + 1 equals the number of anti-divisors of p + 1 : 2, 103, 137, 257,...

Numbers whose sum of proper divisors equals the sum of their anti-divisors: 1, 5, 41,...

LINKS

Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

FORMULA

{n: A032741(n) = A066272(n)}.

EXAMPLE

28 is here since it has 5 proper divisors {2, 4, 7, 14, 28} and 5 anti-divisors {3, 5, 8, 11, 19}.

MAPLE

for n from 1 to 700 do

    if A032741(n) = A066272(n) then

        printf("%d, ", n) ;

    end if;

end do: # R. J. Mathar, Feb 03 2013

PROG

(PARI) is(n)=numdiv(2*n+1)+numdiv(2*n-1)+numdiv(n>>valuation(n, 2))-numdiv(n)==4 || n==1 \\ Charles R Greathouse IV, Feb 04 2013

CROSSREFS

Cf. A000005, A032741, A066272, A073694, A178029.

Sequence in context: A108865 A319497 A036119 * A229269 A050852 A066887

Adjacent sequences:  A211182 A211183 A211184 * A211186 A211187 A211188

KEYWORD

nonn

AUTHOR

Juri-Stepan Gerasimov, Feb 02 2013

EXTENSIONS

Entries corrected by R. J. Mathar, Feb 03 2013

STATUS

approved

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Last modified September 28 08:33 EDT 2020. Contains 337394 sequences. (Running on oeis4.)