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A200519 Least m>0 such that n = y^2 - 9^x (mod m) has no solution, or 0 if no such m exists. 1
0, 4, 4, 0, 8, 4, 4, 0, 0, 4, 4, 9, 8, 4, 4, 0, 0, 4, 4, 0, 8, 4, 4, 9, 0, 4, 4, 0, 8, 4, 4, 80, 9, 4, 4, 0, 8, 4, 4, 63, 0, 4, 4, 80, 8, 4, 4, 9, 0, 4, 4, 45, 8, 4, 4, 0, 9, 4, 4, 9, 8, 4, 4, 0, 133, 4, 4, 80, 8, 4, 4, 15, 0, 4, 4, 63, 8, 4, 4 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

To prove that an integer n is in A051211, it is sufficient to find integers x,y such that y^2 - 9^x = n. In that case, a(n)=0. To prove that n is *not* in A051211, it is sufficient to find a modulus m for which the (finite) set of all possible values of 9^x and y^2 (mod m) allows us to deduce that y^2 - 9^x can never equal n. The present sequence lists the smallest such m>0, if it exists.

LINKS

Table of n, a(n) for n=0..78.

EXAMPLE

See A200512 for motivation and detailed examples.

PROG

(PARI) A200519(n, b=9, p=3)={ my( x=0, qr, bx, seen ); for( m=3, 9e9, while( x^p < m, issquare(b^x+n) & return(0); x++); qr=vecsort(vector(m, y, y^2-n)%m, , 8); seen=0; bx=1; until( bittest(seen+=1<<bx, bx=bx*b%m), for(i=1, #qr, qr[i]<bx & next; qr[i]>bx & break; next(3))); return(m))}

CROSSREFS

Cf. A051204-A051221, A200505-A200520.

Sequence in context: A227033 A285050 A262949 * A129507 A236922 A021698

Adjacent sequences:  A200516 A200517 A200518 * A200520 A200521 A200522

KEYWORD

nonn

AUTHOR

M. F. Hasler, Nov 18 2011

STATUS

approved

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Last modified May 13 05:02 EDT 2021. Contains 343836 sequences. (Running on oeis4.)