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A014574 Average of twin prime pairs. 289
4, 6, 12, 18, 30, 42, 60, 72, 102, 108, 138, 150, 180, 192, 198, 228, 240, 270, 282, 312, 348, 420, 432, 462, 522, 570, 600, 618, 642, 660, 810, 822, 828, 858, 882, 1020, 1032, 1050, 1062, 1092, 1152, 1230, 1278, 1290, 1302, 1320, 1428, 1452, 1482, 1488, 1608 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

With an initial 1 added, this is the complement of the closure of {2} under a*b+1 and a*b-1. - Franklin T. Adams-Watters, Jan 11 2006

Also the square root of the product of twin prime pairs + 1. Two consecutive odd numbers can be written as 2k+1,2k+3. Then (2k+1)(2k+3)+1 = 4(k^2+2k+1) = 4(k+1)^2, a perfect square. Since twin prime pairs are two consecutive odd numbers, the statement is true for all twin prime pairs. - Cino Hilliard, May 03 2006

Or, single (or isolated) composites. Nonprimes k such that neither k-1 nor k+1 is nonprime. - Juri-Stepan Gerasimov, Aug 11 2009

Numbers n such that sigma(n-1) = phi(n+1). - Farideh Firoozbakht, Jul 04 2010

Solutions of the equation (n-1)'+(n+1)'=2, where n' is the arithmetic derivative of n. - Paolo P. Lava, Dec 18 2012

Aside from the first term in the sequence, all remaining terms have digital root 3, 6, or 9. - J. W. Helkenberg, Jul 24 2013

Numbers n such that n^2-1 is a semiprime. - Thomas Ordowski, Sep 24 2015

REFERENCES

Archimedeans Problems Drive, Eureka, 30 (1967).

LINKS

T. D. Noe, Table of n, a(n) for n = 1..10000

C. K. Caldwell, The Prime Glossary: Twin primes

C. K. Caldwell, The Top Twenty: Twin Primes

Y. Fujiwara, Parsing a Sequence of Qubits, IEEE Trans. Information Theory, 59 (2013), 6796-6806.

Y. Fujiwara, Parsing a Sequence of Qubits, arXiv:1207.1138 [quant-ph], 2012-2013.

Eric Weisstein's World of Mathematics, Twin Primes

FORMULA

a(n) = (A001359(n) + A006512(n))/2 = 2*A040040(n) = A054735(n)/2 = A111046(n)/4.

a(n) = A129297(n+4). - Reinhard Zumkeller, Apr 09 2007

A010051(a(n) - 1) * A010051(a(n) + 1) = 1. Reinhard Zumkeller, Apr 11 2012

a(n) = 6*A002822(n-1), n>=2. - Ivan N. Ianakiev, Aug 19 2013

a(n)^4 - 4*a(n)^2 = A062354(a(n)^2 - 1). - Raphie Frank, Oct 17 2013

MAPLE

P := select(isprime, [$1..1609]): map(p->p+1, select(p->member(p+2, P), P)); # Peter Luschny, Mar 03 2011

A014574 := proc(n) option remember; local p ; if n = 1 then 4 ; else p := nextprime( procname(n-1) ) ; while not isprime(p+2) do p := nextprime(p) ; od ; return p+1 ; end if ; end proc: # R. J. Mathar, Jun 11 2011

MATHEMATICA

Select[Table[Prime[n] + 1, {n, 260}], PrimeQ[ # + 1] &] (* Ray Chandler, Oct 12 2005 *)

Mean/@Select[Partition[Prime[Range[300]], 2, 1], Last[#]-First[#]==2&] (* Harvey P. Dale, Jan 16 2014 *)

PROG

(PARI) p=2; forprime(q=3, 1e4, if(q-p==2, print1(p+1", ")); p=q) \\ Charles R Greathouse IV, Jun 10 2011

(Maxima) A014574(n) := block(

    if n = 1 then

        return(4),

    p : A014574(n-1) ,

    for k : 2 step 2 do (

        if primep(p+k-1) and primep(p+k+1) then

            return(p+k)

    )

)$ /* R. J. Mathar, Mar 15 2012 */

(Haskell)

a014574 n = a014574_list !! (n-1)

a014574_list = [x | x <- [2, 4..], a010051 (x-1) == 1, a010051 (x+1) == 1]

-- Reinhard Zumkeller, Apr 11 2012

(GAP) a:=1+Filtered([1..2000], p->IsPrime(p) and IsPrime(p+2)); # Muniru A Asiru, May 20 2018

CROSSREFS

Cf. A000010, A000203, A001359, A002822, A006512, A037074, A040040, A054735, A077800, A111046.

Sequence in context: A280469 A072570 A217259 * A258838 A034425 A073123

Adjacent sequences:  A014571 A014572 A014573 * A014575 A014576 A014577

KEYWORD

nonn,easy,nice,changed

AUTHOR

R. K. Guy, N. J. A. Sloane, Eric W. Weisstein

EXTENSIONS

Offset changed to 1 by R. J. Mathar, Jun 11 2011

STATUS

approved

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Last modified May 25 15:12 EDT 2018. Contains 304562 sequences. (Running on oeis4.)