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A225801 The only semiprime s such that 2*a < s < 2*b where a, b are consecutive semiprimes. 1
51, 69, 74, 77, 106, 111, 115, 129, 187, 226, 235, 237, 267, 291, 314, 355, 371, 403, 407, 411, 422, 427, 437, 497, 519, 533, 591, 597, 649, 655, 669, 713, 723, 763, 785, 789, 831, 893, 905, 939, 943, 965, 1003, 1027, 1037, 1059, 1067, 1073, 1126, 1159, 1165 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Alois P. Heinz, Table of n, a(n) for n = 1..1000

EXAMPLE

74 is the only semiprime in the interval [2*35, 2*38} = [70, 76] where 35, 38 are consecutive semiprimes.

PROG

(PARI) issemi(n)=bigomega(n)==2

precsemi(n)=while(!issemi(n--), ); n

nextsemi(n)=while(!issemi(n++), ); n

is(n)=if(!issemi(n), return(0)); my(a=precsemi(ceil(n/2)), b=nextsemi(n\2)); nextsemi(a)==b && nextsemi(2*a)==n && precsemi(2*b)==n \\ Charles R Greathouse IV, Mar 18 2014

CROSSREFS

Cf. A001358, A166251, A217561.

Sequence in context: A118147 A015863 A232105 * A242045 A113285 A050698

Adjacent sequences:  A225798 A225799 A225800 * A225802 A225803 A225804

KEYWORD

nonn

AUTHOR

Irina Gerasimova, Aug 18 2013

STATUS

approved

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Last modified February 20 12:12 EST 2018. Contains 299387 sequences. (Running on oeis4.)