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 A105720 Triangular matchstick numbers in the class of prime numbers: sum of n-th and next n primes. 7
 5, 15, 36, 67, 112, 169, 240, 323, 424, 539, 662, 803, 964, 1133, 1312, 1523, 1746, 1987, 2246, 2519, 2808, 3119, 3436, 3787, 4154, 4529, 4920, 5337, 5770, 6219, 6682, 7173, 7672, 8203, 8760, 9323, 9912, 10517, 11140, 11783, 12450, 13135, 13836 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Terms are squares at only(?) three values of n = 3, 6, 4072: corresponding terms are 6^2, 13^2, and 15735^2. Terms are prime at many values of n; at n = 1, 4, 16, 18, 22, 36, 40, 44, 52 they are 5, 67, 1523, 1987, 3119, 9323, 11783, 14551, 21019. LINKS Harvey P. Dale, Table of n, a(n) for n = 1..1000 FORMULA a(n) = p(n) + p(n+1) + ... + p(2n-1) + p(2n), where p(k)=k-th prime. a(1)=5; for n > 1, a(n) = a(n-1) - prime(n-1) + prime(2*n-1) + prime(2*n). - Zak Seidov, Oct 18 2009 MATHEMATICA a[n_]:=Plus@@Prime[Range[n, 2n]] a=5; s={5}; Do[a=a-Prime[n]+Prime[2n+1]+Prime[2n+2]; AppendTo[s, a], {n, 10^5}]; (* Zak Seidov, Oct 18 2009 *) Table[Total[Prime[Range[n, 2n]]], {n, 50}] (* Harvey P. Dale, Jun 10 2014 *) PROG (Magma) [ &+[ NthPrime(n+i): i in [0..n] ]: n in [1..50] ]; // Bruno Berselli, Jul 08 2011 (PARI) a(n)=my(s=0); forprime(p=prime(n), prime(2*n), s+=p); s \\ Charles R Greathouse IV, Jul 08 2011 CROSSREFS Cf. A045943 (triangular matchstick numbers: 3*n*(n+1)/2). Cf. A045943. Cf. A166619, A166620. - Zak Seidov, Oct 18 2009 Sequence in context: A333932 A065780 A220480 * A174655 A184631 A366971 Adjacent sequences: A105717 A105718 A105719 * A105721 A105722 A105723 KEYWORD nonn AUTHOR Zak Seidov, May 04 2005 STATUS approved

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Last modified April 21 01:42 EDT 2024. Contains 371850 sequences. (Running on oeis4.)