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A116541
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Triangular numbers for which the number of divisors is also a triangular number.
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4
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1, 28, 45, 153, 171, 325, 496, 2016, 3321, 4753, 4950, 7260, 7381, 8256, 11628, 13203, 14196, 20100, 29161, 41616, 56953, 64620, 65341, 73536, 76636, 77028, 89676, 90100, 97461, 101475, 126756, 130816, 150975, 166176, 166753, 179700, 180300
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OFFSET
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1,2
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LINKS
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EXAMPLE
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496 is in the sequence because it is a triangular number (31*32/2) and has 10=4*5/2 divisors (1,2,4,8,16,31,62,124,248,496).
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MAPLE
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with(numtheory): a:=proc(n) local s: s:=tau(n*(n+1)/2): if type(sqrt(1+8*s)/2-1/2, integer)=true then n*(n+1)/2 else fi end: seq(a(n), n=1..750); # Emeric Deutsch, Apr 06 2006
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MATHEMATICA
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Select[Range[600]*Range[2, 601]/2, IntegerQ@ Sqrt[8 DivisorSigma[0, #] + 1] &] (* Robert G. Wilson v, Apr 20 2006 *)
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PROG
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(PARI)
seq(N) = {
my(a = vector(N), n = 1, cnt=0);
while (cnt < N,
my(tn = n*(n+1)/2, d = numdiv(tn), x = (sqrtint(1+8*d)-1)\2);
if (x*(x+1)/2 == d, a[cnt++] = tn); n++);
return(a);
};
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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Luc Stevens (lms022(AT)yahoo.com), Apr 03 2006
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EXTENSIONS
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STATUS
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approved
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