This site is supported by donations to The OEIS Foundation.

 Please make a donation to keep the OEIS running. We are now in our 55th year. In the past year we added 12000 new sequences and reached 8000 citations (which often say "discovered thanks to the OEIS"). We need to raise money to hire someone to manage submissions, which would reduce the load on our editors and speed up editing. Other ways to donate

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A206400 Number of composites of the form n^2 + 1 between two successive primes of this form. 6
 0, 1, 1, 3, 3, 1, 3, 3, 1, 9, 3, 13, 1, 9, 7, 9, 5, 3, 15, 5, 3, 3, 1, 3, 3, 11, 3, 5, 3, 9, 5, 3, 3, 19, 1, 3, 13, 5, 5, 3, 9, 5, 3, 3, 5, 9, 3, 15, 5, 7, 11, 13, 9, 33, 1, 9, 3, 5, 13, 9, 5, 3, 3, 19, 1, 3, 3, 15, 5, 39, 7, 11, 13, 5, 7, 9, 39, 1, 7, 1, 7 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,4 COMMENTS a(n) = number of composites of A134406  between A002496(n) and A002496(n+1). LINKS Michel Lagneau, Table of n, a(n) for n = 1..10000 EXAMPLE a(4) = 3 because there exists 3 composite numbers of the form n^2+1 : {50, 65, 82} between A002496(4) = 37 and A002496(5) = 101. MAPLE i:=0:for n from 2 to 1000 do:x:=n^2+1:if type (x, prime)=true then printf(`%d, `, i):i:=0:else i:=i+1:fi:od: MATHEMATICA cfn2[{a_, b_}]:=Count[Range[a+1, b-1], _?(IntegerQ[Sqrt[#-1]]&)]; cfn2/@ Partition[ Select[Prime[Range[50000]], IntegerQ[Sqrt[#-1]]&], 2, 1] (* Harvey P. Dale, Jan 13 2019 *) PROG (PARI) c=0; for(n=2, 1e9, !ispseudoprime(n^2+1) & c++ & next; print1(c", "); c=0)  \\ M. F. Hasler, Feb 07 2012 CROSSREFS Cf. A002522, A002496, A134406. Sequence in context: A220670 A264526 A138071 * A321129 A278265 A171369 Adjacent sequences:  A206397 A206398 A206399 * A206401 A206402 A206403 KEYWORD nonn AUTHOR Michel Lagneau, Feb 07 2012 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 7 22:29 EST 2019. Contains 329850 sequences. (Running on oeis4.)