A118854 Numbers m such that m-1 and m have the same number of common totatives as m and m+1 have.

2, 3, 8, 21, 24, 27, 45, 75, 93, 105, 117, 123, 147, 165, 213, 309, 315, 333, 357, 387, 453, 525, 555, 573, 627, 636, 693, 717, 729, 765, 795, 843, 915, 933, 957, 1005, 1083, 1125, 1173, 1227, 1323, 1347, 1437, 1467, 1515, 1563, 1575, 1677, 1725, 1755, 1773

Offset

1,1

Comments

A057475(a(n)-1) = A057475(a(n));

it seems that even values are very rare, see A118855.

Links

Table of n, a(n) for n=1..51.

Eric Weisstein's World of Mathematics, Totative

Example

n = 21, the sets of totatives for 21-1, 21 and 21+1:
T(20) = {1,3,7,9,11,13,17,19},
T(21) = {1,2,4,5,8,10,11,13,16,17,19,20},
T(22) = {1,3,5,7,9,13,15,17,19,21},
A057475(20) = #intersect(T(20),T(21)) = #{1,11,13,17,19} = 5,
A057475(20) = #intersect(T(21),T(22)) = #{1,5,13,17,19} = 5,
therefore 21 is a term.

Crossrefs

Sequence in context: A056971 A108125 A175490 * A192151 A137652 A185381

Adjacent sequences: A118851 A118852 A118853 * A118855 A118856 A118857

Keyword

nonn

Author

Reinhard Zumkeller, May 02 2006

Status

approved