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A206708 Numbers k such that sigma(k) = sigma(sigma(k)-k). 4
6, 28, 220, 284, 496, 1184, 1210, 2620, 2924, 5020, 5564, 6232, 6368, 8128, 10744, 10856, 12285, 14595, 17296, 18416, 63020, 66928, 66992, 67095, 69615, 71145, 76084, 79750, 87633, 88730, 100485, 122265, 122368, 123152, 124155, 139815, 141664, 142310, 153176 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

For all k, let s(k) = sigma(k) - k, the aliquot sum function A001065; then this sequence is the set of k such that s(s(k)) = k. - Jeppe Stig Nielsen, Jan 12 2020

LINKS

Giovanni Resta, Table of n, a(n) for n = 1..10000 (first 1000 terms from Amiram Eldar)

Eric Weisstein's World of Mathematics, Perfect Number

Eric Weisstein's World of Mathematics, Amicable Pair

Wikipedia, Perfect number

Wikipedia, Amicable number

FORMULA

Equals {A063990} union {A000396} =  (amicable numbers) union (perfect numbers).

EXAMPLE

220 is in the sequence because sigma(220) = 504, sigma(504 - 220) = sigma(284) = 504.

MAPLE

with(numtheory);

A206708:=proc(q)  local n;

for n from 1 to q do if sigma(n)=sigma(sigma(n)-n) then print(n); fi; od; end:

A206708(10^10); # Paolo P. Lava, Mar 26 2013

MATHEMATICA

Select[Range[10^6], DivisorSigma[1, #]==DivisorSigma[1, DivisorSigma[1, #]-#]&]

PROG

(PARI) isok(k) = if (k != 1, my(sk=sigma(k)); sk == sigma(sk-k)); \\ Michel Marcus, Jun 24 2019

(MAGMA) [k:k in [2..154000]|s eq DivisorSigma(1, s-k) where s is DivisorSigma(1, k)]; // Marius A. Burtea, Jan 13 2020

CROSSREFS

Cf. A000396 (perfect numbers), A063990 (amicable numbers).

Cf. A000203 (sum of divisors), A001065 (sum of proper divisors).

Sequence in context: A169723 A052395 A034660 * A216413 A090898 A134872

Adjacent sequences:  A206705 A206706 A206707 * A206709 A206710 A206711

KEYWORD

nonn

AUTHOR

Michel Lagneau, Feb 11 2012

STATUS

approved

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Last modified February 21 20:43 EST 2020. Contains 332111 sequences. (Running on oeis4.)