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A072868 Numbers n such that sigma(sigma(n)-n) = n. 10
4, 8, 32, 128, 8192, 131072, 524288, 2147483648, 2305843009213693952, 618970019642690137449562112 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Because numbers of the form 1+A000668(.) are a subsequence, 162259276829213363391578010288128 and 170141183460469231731687303715884105728 are also in the sequence (not necessarily the next ones). - Joerg Arndt, Jul 01 2012

LINKS

Table of n, a(n) for n=1..10.

FORMULA

a(n) = 1 + A000668(n) (conjecture), which implies equivalent conjectures stated in A000668.

MAPLE

with(numtheory);

A072868:=proc(q)  local n;

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

A072868(10^10); # Paolo P. Lava, Mar 26 2013

MATHEMATICA

f[n_]:=Plus@@Divisors[Plus@@Divisors[n]-n]==n; lst={}; Do[If[f[n], AppendTo[lst, n]], {n, 9!}]; lst (* Vladimir Joseph Stephan Orlovsky, May 14 2010 *)

PROG

(PARI) isok(n) = sigma(sigma(n)-n) == n; \\ Michel Marcus, May 06 2016

CROSSREFS

Cf. A000043.

Sequence in context: A227295 A034785 A247938 * A075398 A098579 A261196

Adjacent sequences:  A072865 A072866 A072867 * A072869 A072870 A072871

KEYWORD

nonn,more

AUTHOR

Benoit Cloitre, Jul 27 2002

EXTENSIONS

a(6) inserted by Jaroslav Krizek, Jun 29 2012

STATUS

approved

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Last modified June 22 12:24 EDT 2017. Contains 288613 sequences.