This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
First few rows of the triangle: 1; 2, 1; 4, 4, 1; 8, 12, 6, 1; 15, 30, 23, 8, 1; 26, 65, 68, 37, 10, 1; 42, 126, 168, 126, 54, 12, 1; ...
The 5th prime number, 11, appears three times in A051697. Therefore a(5) = 3.
a = {3}; For[n = 2, n < 100, n++, c = 0; For[j = Prime[n - 1], j < Prime[n + 1], j++, If[j < Prime[n], If[Prime[n] - j < j - Prime[n - 1], c++ ], If[Not[Prime[n + 1] - j < j - Prime[n]], c++ ]]]; AppendTo[a, c]]; a
For n=7 a(7)=7. For n=12, reverse(12)=21; a(12)=nextprime(21)=23.
Table[Which[PrimeQ[n],n,PrimeQ[IntegerReverse[n]],IntegerReverse[n], True, NextPrime[ IntegerReverse[ n]]],{n,100}] (* Harvey P. Dale, May 12 2018 *)
rev(n)={d=digits(n); p=""; for(i=1, #d, p=concat(Str(d[i]), p)); return(eval(p))}
i=0; t=vector(200);
findn(n)={if(isprime(n),t[i++]=n, a=rev(n); b=nextprime(a); t[i++]=b); }
for(n=1,200,findn(n)); t
Comments