A343676 Number of n-digit undulating alternating primes.

4, 9, 23, 49, 175, 321, 1189, 3025, 12111, 28492, 113409, 251513, 1068440, 2629980, 11690210, 28498852, 128871588, 298890814, 1309837146

Offset: 1

Author

Chai Wah Wu, Apr 25 2021

Keywords

• nonn
• base
• more

Comments

a(n) is the number of n-digit terms in A343590.

a(n) <= A057333(n).

Crossrefs

Cf. A002385, A030144, A057333, A059168, A343590, A343675.

Programs

Python

def f(w):
    for s in w:
        for t in range(int(s[-1])+1,10,2):
            yield s+str(t)
def g(w):
    for s in w:
        for t in range(1-int(s[-1])%2,int(s[-1]),2):
            yield s+str(t)
def A343676(n):
    c = 0
    for d in '123456789':
        x = d
        for i in range(1,n):
            x = g(x) if i % 2 else f(x)
        c += sum(1 for p in x if isprime(int(p)))
        if n > 1:
            y = d
            for i in range(1,n):
                y = f(y) if i % 2 else g(y)
            c += sum(1 for p in y if isprime(int(p)))
    return c

Extensions

a(18)-a(19) from Martin Ehrenstein, Apr 27 2021