A190308 - OEIS

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A190308

Numbers k such that tau(4k-1) = tau(4k+1) = tau(6k-1) = tau(6k+1) where tau(k) = A000005(k).

1, 3, 18, 31, 36, 45, 87, 136, 145, 225, 275, 357, 418, 428, 460, 505, 512, 528, 539, 549, 640, 663, 728, 768, 838, 855, 876, 886, 910, 942, 960, 995, 1026, 1030, 1047, 1067, 1083, 1125, 1133, 1157, 1191, 1212, 1291, 1300, 1400, 1443, 1470, 1480, 1491, 1539

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,2

LINKS

Nathaniel Johnston, Table of n, a(n) for n = 1..10000

FORMULA

A000005(4*a(n)-1) = A000005(4*a(n)+1) = A000005(6*a(n)-1) = A000005(6*a(n)+1).

MAPLE

with(numtheory): A190308 := proc(n) option remember: local k, t: if(n=1)then return 1: fi: for k from procname(n-1)+1 do t:=tau(4*k-1): if(t=tau(4*k+1) and t=tau(6*k-1) and t=tau(6*k+1))then return k: fi: od: end: seq(A190308(n), n=1..50); # Nathaniel Johnston, May 25 2011

MATHEMATICA

d[n_] := d[n] = DivisorSigma[0, n]; Select[Range[1600], d[4*#-1] == d[4*#+1] == d[6*#-1] == d[6*#+1] &] (* Amiram Eldar, Dec 02 2023 *)

PROG

(PARI) is(n) = {my(d = numdiv(4*n-1)); numdiv(4*n+1) == d && numdiv(6*n-1) == d && numdiv(6*n+1) == d; } \\ Amiram Eldar, Dec 02 2023

CROSSREFS

Cf. A000005, A190276.

Sequence in context: A202359 A118474 A163242 * A161443 A003337 A047714

Adjacent sequences: A190305 A190306 A190307 * A190309 A190310 A190311

KEYWORD

nonn

AUTHOR

Juri-Stepan Gerasimov, May 08 2011

EXTENSIONS

Terms after a(10) from Nathaniel Johnston, May 25 2011

STATUS

approved