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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
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: A316032 A360395 A316033 * A316034 A316035 A316036
KEYWORD
nonn
AUTHOR
Antonio Roldán, Jan 14 2013
STATUS
approved

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Last modified June 14 21:35 EDT 2024. Contains 373401 sequences. (Running on oeis4.)