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.
a(4) = 17 - A072181(4) = 17 - 12 = 5.
stepA072181(k,n)=if(n<3, return(n)); my(f=factor(k), g=factor(n), p=Set(concat(f[,1],g[,1])), x=((f,p) -> my(i=setsearch(f[,1]~,p)); if(i,f[i,2],1)), e=apply(q->x(f,q)*x(g,q),p)); factorback(concat(Mat(p~),e~)) for(n=1,1e4,k=stepA072181(k,n); if(ispseudoprime(k+1),print1(n", "))) \\ Charles R Greathouse IV, Oct 16 2015
a(4) = A072181(4)-7 = 12-7 = 5.
from sympy import factorint, isprime
def afindn(terms):
prev_factors, prevan, prevk, n = dict(), 1, None, 2
for n in range(2, terms+1):
n_factors, an = factorint(n), 1
for pi in set(prev_factors.keys()) | set(n_factors.keys()):
ei = prev_factors[pi] if pi in prev_factors else 1
fi = n_factors[pi] if pi in n_factors else 1
an *= pi**(ei*fi)
if n >= 3:
if an != prevan:
k = 3
while not isprime(an - k): k += 2
else:
k = prevk
print(k, end=", ")
prevk = k
prev_factors, prevan = factorint(an), an
afindn(36) # Michael S. Branicky, Sep 05 2021
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