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A267431
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Indices of Catalan numbers that are not of the form x^2 + y^2 + z^2 where x, y and z are integers.
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0
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10, 24, 37, 43, 46, 48, 49, 51, 69, 87, 96, 97, 102, 103, 109, 114, 117, 120, 133, 157, 170, 175, 187, 190, 192, 193, 198, 207, 226, 240, 241, 243, 261, 285, 300, 308, 332, 344, 351, 356, 360, 375, 384, 385, 390, 404, 411, 414, 415, 420, 424, 445, 450, 459, 462, 477, 480, 481
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OFFSET
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1,1
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COMMENTS
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Corresponding Catalan numbers are 16796, 1289904147324, 45950804324621742364, 150853479205085351660700, ...
It is obvious that minimum value of a(n) - a(n-1) is 1. Is there a maximum value of a(n) - a(n-1)?
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LINKS
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EXAMPLE
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10 is a term because the 10th Catalan number is 16796 and there are no integer values of x, y and z for the equation 16796 = x^2 + y^2 + z^2.
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PROG
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(PARI) isA004215(n) = { my(fouri, j) ; fouri=1 ; while( n >=7*fouri, if( n % fouri ==0, j= n/fouri -7 ; if( j % 8 ==0, return(1) ) ; ) ; fouri *= 4 ; ) ; return(0) ; } { for(n=1, 400, if(isA004215(n), print1(n, ", ") ; ) ; ) ; }
c(n) = binomial(2*n, n)/(n+1);
for(n=0, 1e3, if(isA004215(c(n)), print1(n, ", ")));
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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