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 A063840 Numbers k such that sigma(k) - usigma(k) is a square and sets a new record for such squares. 1
 1, 18, 28, 40, 84, 120, 208, 372, 424, 552, 624, 900, 1152, 1848, 2784, 3904, 4488, 4992, 7448, 7896, 9120, 11712, 13720, 14280, 19920, 22344, 26040, 27144, 37920, 41160, 52920, 88920, 107100, 132480, 186732, 195300, 208152, 240240, 243432 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS Harry J. Smith, Table of n, a(n) for n = 1..100 PROG (PARI) a=-1; u(n) = sumdiv(n, d, if(gcd(d, n/d)==1, d)); j=[]; for(n=1, 300000, x=sigma(n)-u(n); if(issquare(x), b=x; if(b>a, a=b; j=concat(j, n)))); j (PARI) u(n) = sumdiv(n, d, if(gcd(d, n/d)==1, d)) { n=0; a=-1; for (m=1, 10^9, if(issquare(b=sigma(m) - u(m)), if(b>a, a=b; write("b063840.txt", n++, " ", m); if (n==100, break))) ) } \\ Harry J. Smith, Sep 01 2009 CROSSREFS Cf. A048146. Sequence in context: A171221 A216259 A117101 * A180117 A167333 A259642 Adjacent sequences:  A063837 A063838 A063839 * A063841 A063842 A063843 KEYWORD nonn AUTHOR Jason Earls, Aug 25 2001 STATUS approved

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Last modified October 21 09:09 EDT 2020. Contains 337911 sequences. (Running on oeis4.)