A225801 The only semiprime s such that 2*a < s < 2*b where a, b are consecutive semiprimes.

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

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