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A133234 a(n) is least semiprime (not already in list) such that no 3-term subset forms an arithmetic progression. 1
4, 6, 9, 10, 15, 22, 25, 33, 39, 49, 55, 58, 82, 86, 87, 93, 111, 118, 121, 122, 134, 145, 185, 194, 201, 202, 206, 215, 237, 247, 274, 287, 298, 299, 303, 305, 314, 334, 335, 358, 362, 386, 446, 447, 454, 471, 482, 497, 502, 527, 529, 537, 553, 554, 562, 614 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

This is to semiprimes A001358 as A131741 is to primes A000040.

LINKS

Table of n, a(n) for n=1..56.

FORMULA

a(1) = 4, a(2) = 6, a(n) = smallest semiprime such that there is no i < j < n with a(n) - a(j) = a(j) - a(i).

MATHEMATICA

NextSemiprime[n_] := Block[{c = n + 1, f = 0}, While[Plus @@ Last /@ FactorInteger[c] != 2, c++ ]; c ]; f[l_List] := Block[{c, f = 0}, c = If[l == {}, 2, l[[ -1]]]; While[f == 0, c = NextSemiprime[c]; If[Intersection[l, l - (c - l)] == {}, f = 1]; ]; Append[l, c] ]; Nest[f, {}, 100] (* Ray Chandler *)

CROSSREFS

Cf. A000040, A001358, A065825, A131741.

Sequence in context: A181564 A122492 A178378 * A111206 A087112 A077554

Adjacent sequences:  A133231 A133232 A133233 * A133235 A133236 A133237

KEYWORD

easy,nonn

AUTHOR

Jonathan Vos Post, Oct 13 2007

EXTENSIONS

More terms from Ray Chandler

STATUS

approved

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Last modified August 28 10:58 EDT 2015. Contains 261120 sequences.