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A054905 Smallest composite x such that sigma(x) + 2n = sigma(x + 2n). 12
434, 305635357, 104, 27, 195556, 65, 12, 39, 20, 56, 916, 80, 212282, 57, 44, 106645, 52, 125 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
a(19) > 4293000000, if it exists. - Jud McCranie, May 25 2000
a(19) > 10^11, if it exists. - Charles R Greathouse IV, Oct 26 2022
LINKS
EXAMPLE
a(5) corresponds to n=3+2=5, d=2n=10 and the smallest composite integer is 195556. The next solution is 1152136225.
PROG
(PARI) a(n)=forcomposite(x=3, 10^66, if(sigma(x)+2*n==sigma(x+2*n), return(x)));
for(n=1, 66, print1(a(n), ", ")); \\ Joerg Arndt, Nov 15 2014
(PARI) a19(lim, startAt=39)=startAt=ceil(startAt); my(v=vectorsmall(38), i=(startAt-1)%38); forfactored(n=startAt, lim\1+38, my(t=sigma(n)); if(i++>38, i=1); if(t==v[i]+38, return(n[1]-38)); v[i]=if(vecsum(n[2][, 2])>1, t, 0)) \\ Charles R Greathouse IV, Oct 25 2022
CROSSREFS
Sequence in context: A050507 A145318 A054987 * A160353 A268475 A212227
KEYWORD
nonn,more
AUTHOR
Labos Elemer May 23 2000
EXTENSIONS
Description corrected by Jud McCranie, May 25 2000
STATUS
approved

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Last modified April 14 00:20 EDT 2024. Contains 371652 sequences. (Running on oeis4.)