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A214089 Least prime p such that the first n primes divide p^2-1. 7
3, 5, 11, 29, 419, 1429, 1429, 315589, 1729001, 57762431, 1724478911, 6188402219, 349152569039, 1430083494841, 390499187164241, 1010518715554349, 18628320726623609, 522124211958421799, 522124211958421799, 5936798290039408015951, 311263131154464891496249 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

(a(n)^2 - 1) / A002110(n) is congruent to 0 (mod 4). (a(n)^2 - 1) / (4*A002110(n)) = A215085(n). [J. Stauduhar, Aug 03 2012]

a(n) == +1 or -1 (mod prime(i)) for every i=1,2,...,n. The system of congruences x == +1 or -1 (mod prime(i)), i=1,2,...,n, has 2^(n-1) solutions modulo A002110(n) so that a(n) represents the smallest prime in the corresponding residue classes, allowing efficient computation (see PARI program). - Max Alekseyev, Aug 22 2012

LINKS

Max Alekseyev, Table of n, a(n) for n = 1..32

EXAMPLE

a(5) = 419: 419^2-1 = 175560 = 2^3*3*5*7*11*19 contains the first 5 primes.

a(7) = 1429:  1428=2^2*3*7*17, 1430=2*5*11*13 contains the first 7 primes.

a(8) = 315589: 315589^2-1 = 2^3*3*5*7*11*13*17^2*19*151 contains the first 8 primes.

MAPLE

A214089 := proc(n)

     local m, k, p;

   m:= 2*mul(ithprime(j), j=1..n);

   for k from 1 do

     p:= sqrt(m*k+1);

     if type(p, integer) and isprime(p) then return(p)

     end if

   end do

end proc;

# Robert Israel, Aug 19 2012

MATHEMATICA

f[n_] := Block[{k = 1, p = Times @@ Prime@Range@n}, While[! IntegerQ@Sqrt[4 k*p + 1], k++]; Block[{j = k}, While[! PrimeQ[Sqrt[4 j*p + 1]], j++]; Sqrt[4 j*p + 1]]]; Array[f, 10] (* J. Stauduhar, Aug 18 2012 *)

PROG

(PARI) A214089(n) = {

        local(a, k=4, p) ;

        a=prod(j=1, n, prime(j)) ;

        while(1,

                if( issquare(k*a+1, &p),

                        if(isprime(p),

                                return(p);

                        ) ;

                ) ;

                k+=4;

        ) ;

} ;

(PARI) { a(n) = local(B, q); B=prod(i=1, n, prime(i))^2; forvec(v=vector(n-1, i, [0, 1]), q=chinese(concat(vector(n-1, i, Mod((-1)^v[i], prime(i+1))), [Mod(1, 2)])); forstep(s=lift(q), B-1, q.mod, if(ispseudoprime(s), B=s; break)) ); B } /* Max Alekseyev */

CROSSREFS

Cf. A073917, A103783.

Sequence in context: A265784 A146243 A262936 * A108259 A093933 A165572

Adjacent sequences:  A214086 A214087 A214088 * A214090 A214091 A214092

KEYWORD

nonn,hard

AUTHOR

Robin Garcia, Jul 02 2012

EXTENSIONS

a(15)-a(16) from Donovan Johnson, Jul 25 2012

a(17) from Charles R Greathouse IV, Aug 08 2012

a(18) from Charles R Greathouse IV, Aug 16 2012

a(19) from J. Stauduhar, Aug 18 2012

a(20)-a(32) from Max Alekseyev, Aug 22 2012

STATUS

approved

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Last modified December 5 12:51 EST 2020. Contains 338947 sequences. (Running on oeis4.)