login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A234970 Deficient numbers whose aliquot sequence is deficient, abundant, deficient, ..., etc. 1
284, 1210, 1336, 2122, 2362, 2924, 5234, 5564, 6368, 10856, 12458, 13923, 14595, 18416, 34586, 36843, 66992, 71145, 74385, 76084, 80745, 85939, 87633, 88730, 89228, 90153, 91322, 91792, 123152, 124155, 139815, 153176, 156122, 163148, 168730, 171428, 172166 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
All larger members of an amicable pair (A002046) belong to this sequence.
Also deficient members of the sociable quadruple represented in A222977 are here.
Starting at k=3, I found 1, 9, 28, 93, 266, 774, 2821 terms up to 10^k.
LINKS
EXAMPLE
The aliquot sequence 284->220->284->... has the requested form, so 284 is here.
2122 is here too, since its aliquot sequence is 2122->1064->1336->1184->1210->... .
PROG
(PARI) isAmicable(n)={my(a=sigma(n)-n); (a<>n) && (sigma(a)-a)==n; } \\ from A063990
isSociableADAD(n)={my(a=sigma(n)-n); if (!a, return (0)); my(b=sigma(a)-a); if(! b, return (0)); my(c=sigma(b)-b); if (!c, return (0)); my(d=sigma(c)-c); if (d != n, return (0)); ((n>a) && (a<b) && (b>c) && (c<n)) || ((n<a) && (a>b) && (b<c) && (c>n)); }
isok(n) = {my(oldn = n); my(newn = sigma(oldn) - oldn); my(dir = sign(newn - oldn)); if (!dir || (dir > 0), return (0)); oldn = newn; while (1, newn = sigma(oldn) - oldn; ndir = sign(newn - oldn); if (!ndir || (ndir == dir), return (0)); if (isAmicable(oldn), return(1)); if (isSociableADAD(oldn), return(1)); oldn = newn; dir = ndir; ); }
CROSSREFS
Sequence in context: A259996 A092681 A090789 * A260087 A002046 A339679
KEYWORD
nonn
AUTHOR
Michel Marcus, Jan 02 2014
STATUS
approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified March 1 03:27 EST 2024. Contains 370430 sequences. (Running on oeis4.)