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A259124 If n is representable as x*y+x+y, with x>=y>1, then a(n) is the sum of all x's and y's in all such representations. Otherwise a(n)=0. 4

%I

%S 0,0,0,0,0,0,0,4,0,0,5,0,0,6,6,0,7,0,7,8,0,0,17,8,0,10,9,0,20,0,10,12,

%T 0,10,34,0,0,14,23,0,26,0,13,28,0,0,43,12,13,18,15,0,32,14,29,20,0,0,

%U 67,0,0,36,32,16,38,0,19,24,32,0,76,0,0,44,21,16,44,0,57,44

%N If n is representable as x*y+x+y, with x>=y>1, then a(n) is the sum of all x's and y's in all such representations. Otherwise a(n)=0.

%C The sequence of numbers that never appear in a(n) begins: 1, 2, 3, 11, 27, 35, 51, 53, 79, 83, 89, 93, 117, 123, 125, 135, 143, 145.

%C The indices n at which a(n)=0 are in A254636. - _Vincenzo Librandi_, Jul 16 2015

%H Charles R Greathouse IV, <a href="/A259124/b259124.txt">Table of n, a(n) for n = 1..10000</a>

%F a(n) = Sum({d: d | n+1 and 3 <= d <= sqrt(n+1)}, d + (n+1)/d - 2). - _Robert Israel_, Aug 05 2015

%e 11 = 3*2 + 3 + 2, so a(11)=5.

%p f:= proc(n) local D,d;

%p D:= select(t -> (t >= 3 and t^2 <= n+1), numtheory:-divisors(n+1));

%p add(d + (n+1)/d - 2, d = D);

%p end proc:

%p map(f, [$1..100]); # _Robert Israel_, Aug 05 2015

%o (Python)

%o TOP = 100

%o a = [0]*TOP

%o for y in xrange(2, TOP/2):

%o for x in xrange(y, TOP/2):

%o n = x*y + x + y

%o if n>=TOP: break

%o a[n] += x+y

%o print a

%o (PARI) a(n)=sum(y=2,sqrtint(n+1)-1, my(x=(n-y)/(y+1)); if(denominator(x)==1, x+y)) \\ _Charles R Greathouse IV_, Jun 29 2015

%Y Cf. A254636, A255361.

%K nonn

%O 1,8

%A _Alex Ratushnyak_, Jun 18 2015

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Last modified November 18 13:22 EST 2018. Contains 317306 sequences. (Running on oeis4.)