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A175305 a(1)=1. a(n) = the smallest integer > a(n-1) where both a(n)-1 and a(n)+1 are coprime to all earlier terms of the sequence. 1
1, 2, 4, 6, 12, 18, 24, 30, 42, 60, 72, 102, 108, 138, 150, 168, 180, 192, 198, 228, 240, 270, 282, 312, 348, 420, 432, 462, 522, 570, 600, 618, 642, 660, 810, 822, 828, 858, 882, 1020, 1032, 1050, 1062, 1092, 1152, 1230, 1278, 1290, 1302, 1320, 1368, 1428 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Every term from a(4) on is a multiple of 6.

LINKS

Table of n, a(n) for n=1..52.

MAPLE

A175305 := proc(n) option remember ; local wrks ; if n = 1 then 1; else for a from procname(n-1)+1 do wrks := true; for i from 1 to n-1 do if gcd(a-1, procname(i)) <> 1 or gcd(a+1, procname(i)) <> 1 then wrks := false; end if; end do: if wrks then return a; end if; end do: end if; end proc: seq(A175305(n), n=1..80) ; [From R. J. Mathar, Apr 15 2010]

CROSSREFS

Sequence in context: A239954 A167706 A072121 * A171923 A060735 A181416

Adjacent sequences:  A175302 A175303 A175304 * A175306 A175307 A175308

KEYWORD

nonn

AUTHOR

Leroy Quet, Apr 11 2010

EXTENSIONS

More terms from R. J. Mathar, Apr 15 2010

STATUS

approved

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Last modified June 29 01:46 EDT 2017. Contains 288856 sequences.