Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #36 Oct 24 2024 05:40:42
%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)={my(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