The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 56th year, we are closing in on 350,000 sequences, and we’ve crossed 9,700 citations (which often say “discovered thanks to the OEIS”).

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A238268 The number of unordered ways that n can be written as the sum of two numbers of the form p or 2p, where p is prime. 2
 1, 1, 2, 2, 3, 3, 3, 2, 3, 3, 3, 3, 4, 4, 3, 3, 4, 4, 3, 3, 5, 4, 4, 4, 5, 4, 4, 3, 4, 6, 4, 3, 7, 4, 3, 5, 6, 5, 5, 5, 6, 7, 4, 4, 9, 5, 5, 7, 6, 5, 5, 4, 5, 7, 4, 3, 10, 4, 4, 8, 8, 7, 7, 5, 6, 8, 5, 4, 10, 5, 5, 9, 8, 7, 8, 5, 7, 9, 5, 4, 13, 8, 6, 8, 8, 7 (list; graph; refs; listen; history; text; internal format)
 OFFSET 4,3 COMMENTS p and 2p are terms of A001751. Sequence defined for n >= 4. It is conjectured that all terms of this sequence are greater than zero. LINKS Lei Zhou, Table of n, a(n) for n = 4..10000 EXAMPLE n=4, 4=2+2, one case found. So a(4)=1; ... n=24, 24 = 2+2*11 = 5+19 = 7+17 = 2*5+2*7 = 11+13, 5 cases found.  So a(24)=5; ... n=33, 33 = 2+31 = 2*2+29 = 7+2*13 = 2*5+23 = 11+2*11 = 2*7+19, 6 cases found.  So a(33)=6. MATHEMATICA Table[ct = 0; Do[If[((PrimeQ[i]) || (PrimeQ[i/2])) && ((PrimeQ[n - i]) || (PrimeQ[(n - i)/2])), ct++], {i, 2, Floor[n/2]}]; ct, {n, 4, 89}] PROG (PARI) isp(i) = isprime(i) || (((i % 2) == 0) && isprime(i/2)); a(n) = sum(i=1, n\2, isp(i) && isp(n-i)); \\ Michel Marcus, Mar 07 2014 CROSSREFS Cf. A001751, A002375, A103151. Sequence in context: A108133 A341120 A332252 * A194883 A328404 A175453 Adjacent sequences:  A238265 A238266 A238267 * A238269 A238270 A238271 KEYWORD nonn AUTHOR Lei Zhou, Feb 21 2014 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified December 9 03:23 EST 2021. Contains 349625 sequences. (Running on oeis4.)