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A103533 Even semiprimes of the form prime(n)*prime(n+1) - 1. 7
14, 34, 142, 898, 1762, 5182, 19042, 79522, 213442, 359998, 412162, 627238, 685582, 777922, 1192462, 1299478, 1695202, 2005006, 2585662, 2663398, 3849322, 4536898, 5143822, 5588446, 5673922, 6594502, 7225342, 8363638, 8538058, 12110278 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

5 is the only odd number of the form prime(n)*prime(n+1) - 1. - Klaus Brockhaus, Mar 29 2005

2*A086870(n) is a subsequence of this sequence. They first differ when 313619 is not in A086870, but 2*313619 = 627238 = a(12). This is because 787 and 797 are the first such pair of consecutive primes that are not twins and (787*797-1)/2 is prime. - Jason Kimberley, Oct 22 2015

LINKS

Ray Chandler, Table of n, a(n) for n = 1..10000

EXAMPLE

a(1)=14 because prime(2)*prime(3)- 1=3*5-1=14=2*7;

a(2)=34 because prime(3)*prime(4)- 1=5*7-1=34=2*17;

a(3)=142 because prime(5)*prime(6)-1=11*13-1=142=2*71.

MATHEMATICA

fQ[n_] := Plus @@ Last /@ FactorInteger[n] == 2; Select[ Prime[ Range[490]]*Prime[ Range[2, 491]] - 1, fQ[ # ] &] (* Robert G. Wilson v, Mar 24 2005 *)

Select[Times@@#-1&/@Partition[Prime[Range[500]], 2, 1], EvenQ[#] && PrimeOmega[ #]==2&] (* Harvey P. Dale, Apr 24 2018 *)

PROG

(PARI) for(n=1, 490, if(bigomega(k=prime(n)*prime(n+1)-1)==2, print1(k, ", "))) \\ Klaus Brockhaus, Mar 24 2005

(MAGMA) [a:n in[2..1000]|IsPrime(a div 2)where a is NthPrime(n)*NthPrime(n+1)-1]; // Jason Kimberley, Oct 22 2015

CROSSREFS

Cf. A001358, A006881, A086870, A103746, A269663.

Sequence in context: A072566 A039449 A120875 * A269663 A046425 A018950

Adjacent sequences:  A103530 A103531 A103532 * A103534 A103535 A103536

KEYWORD

easy,nonn

AUTHOR

Giovanni Teofilatto, Mar 22 2005

EXTENSIONS

More terms from Robert G. Wilson v and Klaus Brockhaus, Mar 24 2005

STATUS

approved

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Last modified April 3 22:34 EDT 2020. Contains 333207 sequences. (Running on oeis4.)