A203468 Numbers that have a unique triangular proper divisor greater than 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

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)={my(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

Adjacent sequences: A203465 A203466 A203467 * A203469 A203470 A203471

Keyword

nonn

Author

Antonio Roldán, Jan 14 2013

Status

approved