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 A203468 Numbers that have a unique triangular proper divisor greater than 1. 1
 6, 9, 15, 20, 21, 27, 33, 39, 40, 50, 51, 56, 57, 69, 70, 80, 81, 87, 93, 99, 100, 111, 112, 117, 123, 129, 130, 141, 153, 159, 160, 170, 171, 177, 182, 183, 190, 196, 200, 201, 207, 213, 219, 224, 230, 237, 243, 249, 250, 260, 261, 267, 272, 275, 279, 290 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Antonio Roldán, Table of a(n) for a(n) <= 10^4 EXAMPLE 40 has a unique triangular proper divisor greater than 1: 10. PROG (PARI) istriang(x)=issquare(8*x+1) numpropdivtriang(n)=m=0; for(i=3, n/2, if(istriang(i)&&n/i==n\i, m+=1)); return(m)} {t=0; for(n=1, 200, k=numpropdivtriang(n); if(k==1, t+=1; write("B203468.txt", t, " ", n)))} CROSSREFS Cf. A076170. Sequence in context: A020648 A269416 A020147 * A164383 A168544 A001748 Adjacent sequences:  A203465 A203466 A203467 * A203469 A203470 A203471 KEYWORD nonn AUTHOR Antonio Roldán, Jan 14 2013 STATUS approved

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