login
This site is supported by donations to The OEIS Foundation.

 

Logo

Annual Appeal: Please make a donation (tax deductible in USA) to keep the OEIS running. Over 5000 articles have referenced us, often saying "we discovered this result with the help of the OEIS".

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A178670 Number of ways to express prime(n) as (prime(n+k) + prime(n-k))/2. 6
0, 0, 1, 0, 2, 1, 1, 0, 0, 2, 1, 1, 2, 0, 0, 2, 1, 1, 1, 3, 0, 0, 1, 1, 2, 1, 1, 1, 0, 0, 2, 2, 2, 1, 0, 1, 3, 0, 1, 3, 2, 1, 4, 2, 1, 0, 4, 0, 0, 0, 2, 0, 0, 2, 1, 1, 0, 0, 1, 0, 0, 0, 3, 4, 0, 0, 1, 1, 1, 2, 3, 3, 2, 4, 2, 2, 1, 3, 4, 4, 1, 1, 1, 3, 2, 2, 2, 1, 1, 0, 0, 1, 2, 1, 0, 0, 0, 3, 0, 2 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,5

LINKS

T. D. Noe, Table of n, a(n) for n = 1..10000

EXAMPLE

a(5) = 2 because the 5th prime (11) is half the sum of the 7th and 3rd prime (17+5) or half the sum of the 8th and 2nd prime (19+3).

a(8) = 0 because the 8th prime (19) cannot be expressed as (1/2)*(prime(8+k) + prime(8-k)) for any k.

MATHEMATICA

nn=1000; p=Prime[Range[2*nn]]; Table[s=Take[p, n-1] + Reverse[Take[p, {n+1, 2n-1}]]; Count[s, 2*p[[n]]], {n, nn}]

PROG

(PARI) a(n)={s=2*prime(n); a=0; for(i=1, n-1, if(prime(n+i)+prime(n-i)==s, a=a+1)); a}

CROSSREFS

Cf. A000040, A129363, A178609, A178954.

Sequence in context: A223730 A117452 A029412 * A026921 A198568 A232539

Adjacent sequences:  A178667 A178668 A178669 * A178671 A178672 A178673

KEYWORD

nonn

AUTHOR

Juri-Stepan Gerasimov, Jan 04 2011

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 | More pages
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy .

Last modified December 9 17:46 EST 2016. Contains 278985 sequences.