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A138666 Numbers n such that every sum of consecutive positive numbers ending in n is not prime. 5
1, 8, 14, 18, 20, 25, 26, 28, 32, 33, 35, 38, 39, 44, 46, 48, 50, 56, 58, 60, 62, 63, 65, 68, 72, 74, 77, 78, 80, 81, 85, 86, 88, 92, 93, 94, 95, 98, 102, 104, 105, 108, 110, 111, 116, 118, 119, 122, 123, 124, 125, 128, 130, 133, 134, 138, 140, 143, 144, 145, 146, 148 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Also numbers n such that all terms in row n of A087401 are not prime. Also the index of the triangular numbers in A076768. See A087572 for the least prime, if it exists. David Wasserman points out (in A087572) that n is in this sequence if and only if n and 2n-1 are both not prime. This sequence is infinite because 2k^2 is a term for all k>1.

LINKS

T. D. Noe, Table of n, a(n) for n=1..1000

EXAMPLE

8 is in this sequence because 8, 15=7+8, 21=6+7+8, 26=5+6+7+8, 30=4+5+6+7+8, 33=3+4+5+6+7+8, 35=2+3+4+5+6+7+8 and 36=1+2+3+4+5+6+7+8 are all composite.

MATHEMATICA

Select[Range[200], !PrimeQ[ # ] && !PrimeQ[2#-1] &]

Select[Range[150], AllTrue[Accumulate[Reverse[Range[#]]], !PrimeQ[#]&]&] (* The program uses the AllTrue function from Mathematica version 10 *) (* Harvey P. Dale, Mar 18 2017 *)

PROG

(Haskell)

a138666 n = a138666_list !! (n-1)

a138666_list = map (head . tail) $

   filter (all (== 0) . map a010051 . tail) $ drop 2 a087401_tabl

-- Reinhard Zumkeller, Oct 03 2012

CROSSREFS

Cf. A010051.

Sequence in context: A179186 A192045 A084021 * A072835 A096229 A231718

Adjacent sequences:  A138663 A138664 A138665 * A138667 A138668 A138669

KEYWORD

nice,nonn

AUTHOR

T. D. Noe, Mar 26 2008

STATUS

approved

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Last modified December 15 15:44 EST 2018. Contains 318150 sequences. (Running on oeis4.)