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A037074 Numbers that are the product of a pair of twin primes. 58
15, 35, 143, 323, 899, 1763, 3599, 5183, 10403, 11663, 19043, 22499, 32399, 36863, 39203, 51983, 57599, 72899, 79523, 97343, 121103, 176399, 186623, 213443, 272483, 324899, 359999, 381923, 412163, 435599, 656099, 675683, 685583, 736163 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Each entry is the product of p and p+2 where both p and p+2 are prime, i.e., the product of the lesser and greater of a twin prime pair.

Except for the first term, all entries have digital root 8. - Lekraj Beedassy, Jun 11 2004

The above statement follows from p>3 => (p,p+2)=(6k-1,6k+1) => p(p+2)=36k^2-1=8 (mod 9), and A010888=A010878 (mod 9). - M. F. Hasler, Jan 11 2013

Albert A. Mullin states that m is a product of twin primes iff phi(m)*sigma(m) = (m-3)*(m+1), where phi(m) = A000010(m) and sigma(m) = A000203(m). Of course, for a product of distinct primes p*q we know sigma(p*q) = (p+1)*(q+1) and if p, q, are twin primes, say q = p + 2, then sigma(p*q) = (p+1)*(q+1) = (p+1)*(p+3). - Jonathan Vos Post, Feb 21 2006

Also the area of twin prime rectangles. A twin prime rectangle is a rectangle whose sides are components of twin prime pairs. E.g., The twin prime pair (3,5) produces a 3 X 5 unit rectangle which has area 15 square units. - Cino Hilliard, Jul 28 2006

Except for 15, a product of twin primes is of the form 36k^2-1 (cf. A136017, A002822). - Artur Jasinski, Dec 12 2007

A072965(a(n)) = 1; A072965(m) mod A037074(n) > 0 for all m. - Reinhard Zumkeller, Jan 29 2008

The number of terms less than 10^(2n) is A007508(n). - Robert G. Wilson v, Feb 08 2012

Solutions of the equation n'=2*sqrt(n+1), where n' is the arithmetic derivative of n. - Paolo P. Lava, Oct 30 2012

If m is the product of twin primes, then sigma(m) = m + 1 + 2*sqrt(m + 1), phi(m) = m + 1 - 2*sqrt(m + 1). pmin(m) = sqrt(m + 1) - 1, pmax(m) = sqrt(m + 1) + 1. - Wesley Ivan Hurt, Jan 06 2013

Subset of A210503. - Paolo P. Lava, Jan 28 2013

Semiprimes of the form 4*n^2 - 1. - Vincenzo Librandi, Apr 13 2013

REFERENCES

Albert A. Mullin, "Bicomposites, twin primes and arithmetic progression", Abstract 04T-11-48, Abstracts of AMS, Vol. 25, No. 4, 2004, p. 795.

LINKS

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

FORMULA

a(n) = A001359(n)*A006512(n). A000010(a(n))*A000203(a(n)) = (a(n)-3)*(a(n)+1). - Jonathan Vos Post, Feb 21 2006

a(n)=(A014574(n))^2 - 1. a(n+1) = (6*A002822(n))^2 - 1. - Lekraj Beedassy, Sep 02 2006

a(n) = A075369(n) - 1. - Reinhard Zumkeller, Feb 10 2015

EXAMPLE

a(2)=35 because 5*7=35, that is (5,7) is the 2nd pair of twin primes.

MAPLE

ZL:=[]: for p from 1 to 863 do if (isprime(p) and isprime(p+2) ) then ZL:=[op(ZL), (p*(p+2))]; fi; od; print(ZL); # Zerinvary Lajos, Mar 07 2007

for i from 1 to 150 do if ithprime(i+1) = ithprime(i) + 2 then print({ithprime(i)*ithprime(i+1)}); fi; od; # Zerinvary Lajos, Mar 19 2007

MATHEMATICA

s = Select[ Prime@ Range@170, PrimeQ[ # + 2] &]; s(s + 2) (* Robert G. Wilson v, Feb 21 2006 *)

(* For checking large numbers, the following code is better. For instance, we could use the fQ function to determine that 229031718473564142083 is in this sequence. *) fQ[n_] := Block[{fi = FactorInteger[n]}, Last@# & /@ fi == {1, 1} && Differences[ First@# & /@ fi] == {2}]; Select[ Range[750000], fQ] (* Robert G. Wilson v, Feb 08 2012 *)

Times@@@Select[Partition[Prime[Range[500]], 2, 1], Last[#]-First[#]==2&] (* Harvey P. Dale, Oct 16 2012 *)

PROG

(PARI) g(n) = for(x=1, n, if(prime(x+1)-prime(x)==2, print1(prime(x)*prime(x+1)", "))) \\ Cino Hilliard, Jul 28 2006

(MAGMA)  [p*(p+2): p in PrimesUpTo(1000) | IsPrime(p+2)];  // Bruno Berselli, Jul 08 2011

(Haskell)

a037074 = subtract 1 . a075369  -- Reinhard Zumkeller, Feb 10 2015

-- Reinhard Zumkeller, Feb 10 2015, Aug 14 2011

(MAGMA) IsSemiprime:=func<n | &+[d[2]: d in Factorization(n)] eq 2>; [s: n in [1..500] | IsSemiprime(s) where s is 4*n^2-1]; // Vincenzo Librandi, Apr 13 2013

CROSSREFS

Cf. A000010, A000203, A001359, A006512, A014574, A136017, A074480 (multiplicative closure).

Cf. A071700 (subsequence).

Cf. A075369.

Sequence in context: A074480 A194580 A210503 * A107423 A027442 A208728

Adjacent sequences:  A037071 A037072 A037073 * A037075 A037076 A037077

KEYWORD

nice,nonn

AUTHOR

Felice Russo

EXTENSIONS

More terms from Erich Friedman

STATUS

approved

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Last modified August 18 10:48 EDT 2017. Contains 290709 sequences.