cp's OEIS Frontend

A369155 Numbers k such that d(k) < d(k - 1) and d(k) < d(k + 1), and d(k) is also a record for this type of number where d(k) is the number of divisors of k.

%I A369155 #45 Feb 08 2024 20:27:45
%S A369155 5,9,51,153,351,3249,6579,19551,47151,122451,246975,393471,3292289,
%T A369155 10792495,15270849,25770879,58967271,60642945,242340175,481701375,
%U A369155 5122147185,6644739375,6971026699,21061868751,92330654625,213089528575,1159484186575,1305664357375
%N A369155 Numbers k such that d(k) < d(k - 1) and d(k) < d(k + 1), and d(k) is also a record for this type of number where d(k) is the number of divisors of k.
%C A369155 Numbers in A075025 with record number of divisors.
%e A369155 351 is a term in this sequence because d(351) = 8, d(350) = 12, and d(352) = 12, so 351 is a number that has fewer divisors than each of its neighbors, but no number below 351 has that property and has at least 8 divisors.
%o A369155 (PARI) lista(kmax) = my(d1 = numdiv(1), d2 = numdiv(2), d3, dm = 0); for(k = 3, kmax, d3 = numdiv(k); if(d2 < d1 && d2 < d3 && d2 > dm, print1(k-1, ", "); dm = d2); d1 = d2; d2 = d3); \\ _Amiram Eldar_, Jan 16 2024
%Y A369155 Cf. A000005. Subsequence of A075025.
%K A369155 nonn,hard
%O A369155 1,1
%A A369155 _Zhicheng Wei_, Jan 14 2024
%E A369155 a(9)-a(20) from _Michel Marcus_, Jan 15 2024
%E A369155 a(21)-a(22) from _Amiram Eldar_, Jan 16 2024
%E A369155 a(23)-a(28) from _Martin Ehrenstein_, Feb 08 2024