login
a(n) is the least integer that is 4-dimensional pyramidal in exactly n ways.
0

%I #13 Apr 14 2020 05:26:22

%S 5,15,35,140,1820,11375,820820,19019000,10790015600,1568726956160,

%T 7278234628665,7271181889157550

%N a(n) is the least integer that is 4-dimensional pyramidal in exactly n ways.

%C a(n) has exactly n representations as an 4-dimensional pyramidal number P(m, k) = binomial(k + 2, 3)*(k*(m - 2) - m + 6) / 4, with m > 2, k > 1.

%H <a href="/index/Ps#pyramidal_numbers">Index to sequences related to pyramidal numbers</a>

%e a(3) = 35 because 35 is the least integer which is 4-dimensional pyramidal in 3 ways (35 = P(3, 4) = P(7, 3) = P(33, 2)).

%Y Cf. A063778, A080852, A261721, A333916.

%K nonn,more

%O 1,1

%A _Ilya Gutkovskiy_, Apr 10 2020

%E a(9) from _Giovanni Resta_, Apr 11 2020

%E a(9) corrected and a(10)-a(12) from _Bert Dobbelaere_, Apr 14 2020