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 A261057 Number of solutions to c(1)*prime(1)+...+c(2n-1)*prime(2n-1) = -2, where c(i) = +-1 for i > 1, c(1) = 1. 19
 0, 0, 1, 1, 5, 13, 40, 123, 388, 1284, 4332, 14868, 51094, 178361, 634422, 2260717, 8066841, 29030051, 105247340, 383574146, 1404657053, 5171018981, 19140750300, 71124341227, 263546155710, 983417309702, 3684399940711, 13818092760075, 51937827473594, 195956606402526 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,5 COMMENTS There cannot be a solution for an even number of terms on the l.h.s. because there would be an odd number of odd terms but the r.h.s. is even. LINKS Alois P. Heinz, Table of n, a(n) for n = 1..300 FORMULA a(n) = A113041(n) - A022896(2n-1). EXAMPLE a(1) = a(2) = 0 because prime(1) and prime(1) +- prime(2) +- prime(3) is always different from -2. a(3) = 1 because prime(1) - prime(2) - prime(3) - prime(4) + prime(5) = -2. MAPLE s:= proc(n) option remember;       `if`(n<2, 0, ithprime(n)+s(n-1))     end: b:= proc(n, i) option remember; `if`(n>s(i), 0, `if`(i=1, 1,       b(abs(n-ithprime(i)), i-1)+b(n+ithprime(i), i-1)))     end: a:= n-> b(4, 2*n-1): seq(a(n), n=1..30);  # Alois P. Heinz, Aug 08 2015 MATHEMATICA s[n_] := s[n] = If[n<2, 0, Prime[n]+s[n-1]]; b[n_, i_] := b[n, i] = If[n > s[i], 0, If[i == 1, 1, b[Abs[n-Prime[i]], i-1] + b[n+Prime[i], i-1]]]; a[n_] := b[4, 2*n-1];  Table[a[n], {n, 1, 30}] (* Jean-François Alcover, Nov 11 2015, after Alois P. Heinz *) PROG (PARI) A261057(n, rhs=-2, firstprime=1)={rhs-=prime(firstprime); my(p=vector(2*n-2+bittest(rhs, 0), i, prime(i+firstprime))); sum(i=1, 2^#p-1, sum(j=1, #p, (-1)^bittest(i, j-1)*p[j])==rhs)} \\ For illustrative purpose; too slow for n >> 10. (PARI) a(n, s=-2-prime(1), p=1)={if(n<=s, if(s==p, n==s, a(abs(n-p), s-p, precprime(p-1))+a(n+p, s-p, precprime(p-1))), if(s<=0, a(abs(s), max(sum(i=p+1, p+=2*n-2+bittest(s, 0), prime(i)), 1), prime(p))))} \\ M. F. Hasler, Aug 09 2015 CROSSREFS Cf. A261059, A261060, A261045 (starting with prime(2) - prime(4)), A261061 - A261063 and A261044 (r.h.s. = -1), A022894 - A022904, A083309, A022920 (r.h.s. = 0, 1 or 2); A113040, A113041, A113042. Sequence in context: A272585 A026069 A054856 * A283456 A121872 A228922 Adjacent sequences:  A261054 A261055 A261056 * A261058 A261059 A261060 KEYWORD nonn AUTHOR M. F. Hasler, Aug 08 2015 EXTENSIONS a(26)-a(30) from Alois P. Heinz, Jan 04 2019 STATUS approved

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Last modified September 16 08:53 EDT 2019. Contains 327092 sequences. (Running on oeis4.)