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 A203468 Numbers that have a unique triangular proper divisor greater than 1. 1

%I #33 Sep 03 2013 23:49:43

%S 6,9,15,20,21,27,33,39,40,50,51,56,57,69,70,80,81,87,93,99,100,111,

%T 112,117,123,129,130,141,153,159,160,170,171,177,182,183,190,196,200,

%U 201,207,213,219,224,230,237,243,249,250,260,261,267,272,275,279,290

%N Numbers that have a unique triangular proper divisor greater than 1.

%H Antonio Roldán, <a href="/A203468/b203468.txt">Table of a(n) for a(n) <= 10^4</a>

%e 40 has a unique triangular proper divisor greater than 1: 10.

%o (PARI)

%o istriang(x)=issquare(8*x+1)

%o numpropdivtriang(n)=m=0;for(i=3,n/2,if(istriang(i)&&n/i==n\i,m+=1));return(m)}

%o {t=0;for(n=1,200,k=numpropdivtriang(n);if(k==1,t+=1;write("B203468.txt",t," ",n)))}

%Y Cf. A076170.

%K nonn

%O 1,1

%A _Antonio Roldán_, Jan 14 2013

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Last modified July 21 12:49 EDT 2024. Contains 374474 sequences. (Running on oeis4.)