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 A256151 Triangular numbers n such that sigma(n) is a square number. 5
 1, 3, 66, 210, 820, 2346, 4278, 22578, 27966, 32131, 35511, 51681, 53956, 102378, 169653, 173755, 177906, 223446, 241860, 256686, 306153, 310866, 349866, 431056, 434778, 470935, 491536, 512578, 567645, 579426, 688551, 799480, 845650, 893116, 963966, 1031766, 1110795, 1200475, 1613706, 1719585 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS This sequence is the intersection of A000217 and A006532. The corresponding triangular indices are in A116990. - Michel Marcus, Mar 17 2015 LINKS Amiram Eldar, Table of n, a(n) for n = 1..10000 EXAMPLE 3 is in the sequence because 3=2*3/2 is triangular, and sigma(3)=1+3=4=2^2 is square. MATHEMATICA Select[Accumulate[Range[0, 2000]], IntegerQ@Sqrt@DivisorSigma[1, #] &] (* Michael De Vlieger, Mar 17 2015 *) PROG (PARI) {for(i=1, 2*10^3, n=i*(i+1)/2; if(issquare(sigma(n)), print1(n, ", ")))} (MAGMA) [n*(n+1) div 2: n in [1..2000] | IsSquare(SumOfDivisors(n*(n+1) div 2))]; // Vincenzo Librandi, Mar 17 2015 CROSSREFS Cf. A000290, A000217, A006532, A074285, A256149, A256150, A256152. Sequence in context: A028567 A003359 A292064 * A238471 A259457 A157543 Adjacent sequences:  A256148 A256149 A256150 * A256152 A256153 A256154 KEYWORD nonn AUTHOR Antonio Roldán, Mar 16 2015 STATUS approved

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Last modified December 13 23:14 EST 2019. Contains 329974 sequences. (Running on oeis4.)