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 A319035 Triangular numbers T(k) that have the same number of divisors as their successors T(k+1). 5
 6, 10, 15, 66, 153, 406, 435, 561, 861, 903, 1378, 1540, 1770, 2211, 2346, 2556, 2926, 3655, 3916, 4186, 4371, 5151, 5778, 6555, 7626, 9453, 10011, 10296, 11175, 11325, 12720, 14535, 14878, 16110, 16836, 17205, 17391, 17766, 18336, 19306, 19503, 20301, 20706 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Not every term T(k) has the same prime signature as its successor triangular number T(k+1); the first counterexample is the pair (T(52), T(53)) = (1378, 1431) = (2 * 13 * 53, 3^3 * 53), each of which has 8 divisors. The first counterexample in which the two triangular numbers have the same number of distinct prime factors is (T(45630), T(45631)) = (1041071265, 1041116896) = (3^3 * 5 * 13^2 * 45631, 2^5 * 23 * 31 * 45631), each of which has 48 divisors. LINKS Muniru A Asiru, Table of n, a(n) for n = 1..10000 EXAMPLE T(2) = 6 is a term because 6 = 2 * 3 has 4 divisors (1, 2, 3, 6) and T(3) = 10 = 2 * 5 also has 4 divisors (1, 2, 5, 10). T(17) = 153 is a term because 153 = 3^2 * 17 has 6 divisors (1, 3, 9, 17, 51, 153) and T(18) = 171 = 3^2 * 19 also has 6 divisors (1, 3, 9, 19, 57, 171). MATHEMATICA t[n_] := n(n+1)/2; aQ[n_] := DivisorSigma[0, t[n]] == DivisorSigma[0, t[n+1]]; t[Select[Range[100], aQ]] (* Amiram Eldar, Dec 06 2018 *) PROG (PARI) lista(nn) = {for (n=1, nn, if (numdiv(t=n*(n+1)/2) == numdiv((n+1)*(n+2)/2), print1(t, ", ")); ); } \\ Michel Marcus, Dec 06 2018 (GAP) T:=List([1..210], n->n*(n+1)/2);;  a:=List(Filtered([1..Length(T)-1], i->Tau(T[i])=Tau(T[i+1])), k->T[k]); # Muniru A Asiru, Dec 06 2018 CROSSREFS Cf. A000005, A000217, A081978. Cf. A276542 (indices of these triangular numbers). Sequence in context: A332392 A048078 A229276 * A117309 A228300 A299996 Adjacent sequences:  A319032 A319033 A319034 * A319036 A319037 A319038 KEYWORD nonn AUTHOR Jon E. Schoenfield, Dec 05 2018 STATUS approved

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Last modified May 25 23:38 EDT 2020. Contains 334608 sequences. (Running on oeis4.)