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 A057535 Numbers expressible as (a^2 - 1)(b^2 - 1) in 5 distinct ways. 6
 588107520, 67270694400, 546939993600, 2128050512640, 37400697734400, 5566067918611200 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The next term (if it exists) is greater than 2^70. LINKS K. S. Brown, Numbers Expressible as (a^2-1)(b^2-1) K. S. Brown, MathPages Most Wanted List, Problem #14. PROG (PARI) { f(a, b) = (a+1)*(a-1)*(b+1)*(b-1) } ans=vector(6, x, [0, 0]); clear=ans; { g(a) = b=divisors(a*a-1); l=length(b); b=b+vector(l, x, a); for(x=1, l/2, c=4*a*b[x]*(a+1)*(a-1)*(b[x]+1)*(b[x]-1)*(a*b[x]-1)/((b[x]-a)*(b[x]-a)); d=floor(sqrt(sqrt(c))); count=1; for( y=2, d, if (c%(y*y-1)==0, e=ceil(sqrt(c/(y*y-1))); if (f(y, e)==c, ans[count]=[y, e]; count=count+1, ), ); ); if ( count>5, print("g:", a, " ", c, " ", ans); ans=clear, ); ); } { find()= for(n=560, 10001, print(n); g(n)); } Store program as text file, load gp, \r textfilename and then run function find() to search for a 7th entry. (PARI) { f(a, b) = (a+1)*(a-1)*(b+1)*(b-1) } ans=vector(6, x, [0, 0]) CROSSREFS Cf. A063066, A063067, A063068. Cf. A134856, A134857, A134858 (identical?). Sequence in context: A032755 A234391 A297448 * A134858 A145277 A253166 Adjacent sequences:  A057532 A057533 A057534 * A057536 A057537 A057538 KEYWORD nonn AUTHOR K. S. Brown (ksbrown(AT)seanet.com), Fred W. Helenius (fredh(AT)ix.netcom.com), Dean Hickerson, Randall L. Rathbun STATUS approved

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Last modified December 4 15:36 EST 2020. Contains 338928 sequences. (Running on oeis4.)