A236185 Differences between terms of compacting Eratosthenes sieve for prime(4) = 7.

4, 2, 4, 2, 4, 6, 2, 6, 4, 2, 4, 2, 4, 6, 2, 6, 4, 2, 4, 2, 4, 6, 2, 6, 4, 2, 4, 2, 4, 6, 2, 6, 4, 2, 4, 2, 4, 6, 2, 6, 4, 2, 4, 2, 4, 6, 2, 6, 4, 2, 4, 2, 4, 6, 2, 6, 4, 2, 4, 2, 4, 6, 2, 6, 4, 2, 4, 2, 4, 6, 2, 6, 4, 2, 4, 2, 4, 6, 2, 6, 4, 2, 4, 2, 4, 6, 2

Offset

1,1

Comments

P(x) is a function which represents a prime number at a ordinal x.

This pattern, dp(x) is a sequence of the differences between consecutive prime numbers as described by p(x).

For P(4), dp(4)is the relative offsets of the next 7 primes: 7, +4 = 11, +2 = 13, +4 = 17, +2 = 19, +4 = 23, +6 = 29, +2 = 31

The Eratosthenes sieve can be expressed as follows. Start with S1 = [2, 3, 4, 5, ...] the list of numbers bigger than 1. Removing all multiples of the first element 2 yields the list S2 = [3, 5, 7, 9, ...]. Removing all multiples of the first element 3 yields S3 = [5, 7, 11, 13, 17, 19, ...], Removing all multiples of the first element 5 yields S4 = [7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 49, ...], and so on. The list of first differences of S4 is [4, 2, 4, 2, 4, 6, 2, 6, 4, 2, 4, 2, ...] which is this sequence. - Michael Somos, Mar 12 2014

Links

Table of n, a(n) for n=1..87.

Christopher J. Hanson, The structure of prime numbers and twin prime gaps

Formula

a(n + 8) = a(n). - Michael Somos, Mar 10 2014

a(n) = 4*((n+2) mod 2) + 2*((n+1) mod 2) + 4*(f(8,n+2)+f(8,n)) - 2*f(8,n+1), where f(x,n)= floor(n/x)-floor((n-1)/x). - Gary Detlefs, Nov 16 2020

Prog

(PARI) {a(n) = my(A); if( n<1, 0, A = vector( n*28 + 48, k, k+1); for( i = 1, 3, A = select( k -> k%prime(i), A) ); polcoeff( (1 - x) * Ser( select( k -> k>7 && (k%7) == 0, A) / 7), n)) }; /* Michael Somos, Mar 10 2014 */
(C#)
// dp(4) = GeneratePrimeDifferencialPattern( 4 );
static void GeneratePrimeDifferencialPattern( int ordinal )
{
    // Contract
    if( ordinal < 1 )
        throw new ArgumentOutOfRangeException( "ordinal" );
    // Local data
    int size = 1 << 18;
    int[] numberLine = Enumerable.Range( 2, size ).ToArray();
    int pointer = 0;
    // Apply sieve: for each integer greater than 1
    while( pointer < numberLine.Length )
    {
        // Locals
        int x = numberLine[pointer];
        int index = pointer;
        List<int> pattern = new List<int>();
        int skips = 0;
        int count = 0;
        bool counting = true;
        // Find all products
        for( int n = x + x; n < size; n += x )
        {
            // Fast forward through number-line
            while( numberLine[++index] < n )
                skips++;
            // If the number was not already removed
            if( numberLine[index] == n )
            {
                // Add skip count to pattern
                pattern.Add( numberLine[index] );
                // Mark as not prime
                numberLine[index] = 0;
                // Reset skips
                if( counting )
                {
                    count++;
                    if( skips <= 2 )
                        counting = false;
                }
                skips = 0;
            }
            // Otherwise we've skipped again
            else skips++;
        }
        // Reduce number-line
        numberLine = numberLine.Where( n => n > 0 ).ToArray();
        // If we have a pattern we want
        if( pattern.Any() && pointer == ordinal - 1 )
        {
            // Report pattern
            int prime = numberLine[ordinal-1];
            var d = pattern.Take( count ).ToArray();
            List<int> dp = new List<int>();
            for( int y = 1; y < count; y++ )
                dp.Add( ( d[y] - d[y - 1] ) / prime );
            System.Console.WriteLine( "Pattern P({0}) = {1} :: dp({0}) = {2}", pointer + 1, numberLine[pointer], String.Join( ", ", dp ) );
            return;
        }
        // Move number-line pointer forward
        pointer++;
    }
}

Crossrefs

Cf. A236175-A236180, A236185-A236190.

Cf. A007775

Sequence in context: A209272 A105397 A328999 * A300004 A147973 A278527

Adjacent sequences: A236182 A236183 A236184 * A236186 A236187 A236188

Keyword

nonn

Author

Christopher J. Hanson, Jan 21 2014

Extensions

Edited by Michael Somos, Mar 12 2014. (Added code and comments, refined description.)

Status

approved